Endurance

Training and racing for running, cycling and triathlon: intensity zones, heart-rate math, how to split easy and hard work, pacing and race prediction, race-day fueling, heat and altitude, and injury risk. The physiology behind thresholds and fuel is in energy systems, everyday eating in nutrition and hydration, lifting in strength training, load progression and tapering in training plans, and warm-ups in running warm-up and drills.

Intensity zones

Two thresholds anchor every zone system: LT1 (first lactate rise, ≈ 1–2 mmol/L) and LT2 (second, steeper rise, ≈ 2.5–4 mmol/L). The 3-zone model splits at those two points; the Norwegian 5-zone scale subdivides it.

Zone values from the Olympiatoppen I-scale; lactate ranges are mean ± 1 SD in 160 elite runners and cyclists, so use your own tested values where you have them.

ACSM intensity classes (health and fitness)

From Garber et al. 2011 (ACSM position stand). Its weekly guideline is 150 min moderate or 75 min vigorous exercise, or a mix.

Heart rate: HRmax and Karvonen

• Any age formula is off by about ± 11 bpm for one person in three (SEE ≈ 10.8 bpm). Two 40-year-olds can differ by 20+ bpm.
• A measured HRmax is better; ACSM recommends direct measurement when feasible (Garber et al. 2011). A common field method is the highest reading in a hard 3–5 min uphill effort after a full warm-up, or at a 5 k finish.
• HRmax is not changed by training status or sex; it falls about 0.7 bpm per year of age (Tanaka et al. 2001).

Karvonen (heart-rate reserve) method:

with $p$ the fraction of reserve (0.6 for 60% HRR). Measure resting HR on waking, lying down, over several days. Karvonen allows for a low resting HR, so it suits trained people better than plain %HRmax.

Intensity distribution

Practical rule: keep easy days truly easy (conversational, I-1 or I-2), make 1–2 sessions a week hard, and add threshold work (I-3) as the race gets longer.

Watch for Seiler's explanation of why most of the work should be easy and what "hard" days are for.

Key sessions and long runs

Long runs and rides

• Keep each long run within about 10% of your longest run in the past 30 days. In 5 205 runners, single runs more than 10% longer than that raised the overuse-injury rate (HRR 1.64 for 10–30% longer, 2.28 for more than double) (Frandsen et al. 2025).
• Use long runs to practice race fueling and kit: the gut adapts to carbohydrate intake during exercise (Jeukendrup 2017).

Pacing and race prediction

The first km of a race feels easy at any pace; going out 5% too fast costs more later than it gains.

Riegel formula

$T_1$ is a recent race time over distance $D_1$; $T_2$ is the predicted time for $D_2$.

Equivalent race times (Riegel, exponent 1.06)

Treat the marathon column as a best case: for half of recreational runners the real time was at least 10 min slower (Vickers & Vertosick 2016), unless their training included long runs and high weekly volume.

Race fueling and hydration

Daily carbohydrate and recovery eating are in nutrition and hydration. This section covers the race itself.

Carbohydrate

• The amounts are per hour, not per kg; lower them if you are running slowly (a lower absolute intensity burns less carbohydrate) (Jeukendrup 2014).
• High intakes need a trained gut: practice in long runs for weeks before the race (Jeukendrup 2017).
• Before the race: carbohydrate load 10–12 g/kg/day for 36–48 h before events over 90 min; eat 1–4 g/kg in the 1–4 h before the start (Thomas et al. 2016).

Fluid and sodium

Heat and altitude

Heat

From the NHS. Heatstroke can kill: if in doubt, treat it as heatstroke and get medical help.

Altitude

Cycling power and triathlon

Power zones (Coggan)

FTP (functional threshold power) is the highest power you could hold for about an hour. Common field test: 95% of your average power in an all-out 20 min effort after a thorough warm-up (Allen & Coggan). The 95% fits only about half of riders: the real ratio runs from about 86% (sprinters) to 96% (time triallists).

Coggan's levels as published by TrainingPeaks.

Triathlon notes

Watch for how to structure a brick session and settle your cadence off the bike.

Injury risk, form and shoes

Load

Form

Watch for posture, landing under the body and cadence cues rather than forcing a particular foot strike.

Shoes

Strength and concurrent training

Recipes

Three key running sessions

One easy, one threshold and one VO₂max session. Paces use the 30 min time-trial method from energy systems or recent race times.

EASY (I-1 to I-2)
  40–60 min conversational; finish with 4–6 × 20 s strides
  HR ≈ 55–82% HRmax; you could speak in full sentences
 
THRESHOLD (I-3 to I-4)
  15 min easy + drills
  3 × 10 min at ≈ 1 h race pace (≈ 30 min TT pace), 2 min jog
  10 min easy
  Should feel "comfortably hard"; last rep no faster than the first
 
VO₂MAX (I-4 to I-5)
  15 min easy + drills + 3 strides
  5 × 3 min at ≈ 3–5 k race pace, 2–3 min jog
  10 min easy
  Stop if you can't hold pace on two reps

Weekly templates: 5 k, 10 k, half-marathon

About 80% easy by time, two quality sessions, one long run. Lifting fits on quality days (see strength training).

         5 k (4 runs)          10 k (5 runs)           Half (5 runs)
Mon      rest                  rest                    rest
Tue      VO₂max: 5 × 3 min     VO₂max: 5 × 3 min       threshold: 3 × 10 min
Wed      easy 30–40 min        easy 40 min             easy 40–50 min
Thu      threshold: 3 × 8 min  threshold: 3 × 10 min   easy 40 min + strides
Fri      rest                  easy 30 min             rest
Sat      easy 30 min + strides easy 40 min + strides   easy 40 min
Sun      long 60–75 min        long 75–90 min          long 90–120 min, last 20–30 min
                                                       at half pace in later weeks

Build long runs gradually: each within about 10% of the longest in the past 30 days. Taper and full 12-week plans are in training plans.

Marathon race-day fueling plan

For a 3:30–4:30 marathon. Amounts from Thomas et al. 2016 and Jeukendrup 2014; rehearse all of it in long runs.

Day -2 to -1   carbohydrate load 10–12 g/kg/day, low fiber, low fat
               (70 kg: 700–840 g/day, e.g. rice, pasta, bread, juice)
Race morning   2–4 h before: 1–4 g/kg carbohydrate (70 kg: 70–280 g),
               familiar food; optional caffeine 3 mg/kg ≈ 60 min before
Start          sip to thirst; nothing new today
Every 20–30 min
               one gel (check the label: often 20–30 g carbohydrate)
               target 60 g/h; up to 90 g/h glucose + fructose if trained
Drink          to thirst at aid stations, ≈ 0.4–0.8 L/h in mild weather;
               sports drink or salt if sweating heavily
From 30 km     keep taking gels even if you don't feel like it
Finish         fluids with sodium, carbohydrate + protein meal

Karvonen zones, worked

A 50-year-old with resting HR 60.

HRmax (Tanaka)  = 208 − 0.7 × 50  = 173 bpm   (220 − 50 = 170)
HRR             = 173 − 60        = 113 bpm
 
Target = 60 + p × 113
  60% HRR  →  60 + 68  = 128 bpm   easy (ACSM vigorous starts here)
  70% HRR  →  60 + 79  = 139 bpm   upper easy
  80% HRR  →  60 + 90  = 150 bpm   threshold region
  90% HRR  →  60 + 102 = 162 bpm   VO₂max intervals
 
Formula error ≈ ±11 bpm: check against a measured HRmax
and how the pace feels (talk test).

Sweat-rate test

Run this once in each season's weather, then plan race drinking from it (ACSM, Sawka et al. 2007).

1. Pee, then weigh yourself nude before a 60 min run at race effort.
2. Note everything you drink during the run (mL).
3. Towel dry and weigh nude again straight after, before drinking or peeing.
4. Sweat rate (L/h) = (mass before − mass after, in kg) + fluid drunk (L), all over hours run.
5. Aim to replace enough to keep losses under 2% of body mass, never more than you lose.

Before 70.0 kg, after 69.1 kg, drank 0.4 L in 1 h
Sweat rate = (70.0 − 69.1) + 0.4 = 1.3 L/h
Loss = 0.9 kg = 1.3% of body mass: fine for 1 h
3.5–4 h marathon at 1.3 L/h: drink ≈ 0.9–1.0 L/h to stay under 2%

References

• Olympiatoppen: intensity scale (OLT I-scale) (opens in a new tab): 5-zone %HRmax, lactate, RPE and breathing cues
• Seiler-Viken et al.: Contextualizing the Norwegian standardized intensity zone framework (Sci Rep 2025) (opens in a new tab): LT1 and LT2, 3- vs 5-zone models, %HRmax at thresholds
• Bok, Rakovac & Foster: The talk test, feeling scale and RPE (Sports Med 2022) (opens in a new tab): subjective intensity markers
• Garber et al.: ACSM position stand, quantity and quality of exercise (MSSE 2011) (opens in a new tab): intensity classes by %HRR, %HRmax and RPE
• Tanaka, Monahan & Seals: Age-predicted maximal heart rate revisited (JACC 2001) (opens in a new tab): 208 − 0.7 × age
• Nes et al.: Age-predicted maximal heart rate in healthy subjects, the HUNT fitness study (Scand J Med Sci Sports 2013) (opens in a new tab): error of age formulas
• Seiler: What is best practice for training intensity and duration distribution in endurance athletes? (IJSPP 2010) (opens in a new tab): polarised distribution
• Oliveira, Boppre & Fonseca: Polarized versus other training intensity distributions, meta-analysis (Sports Med 2024) (opens in a new tab): polarised vs pyramidal evidence
• Frandsen et al.: How much running is too much? (BJSM 2025) (opens in a new tab): single-session spikes and injury
• Nielsen et al.: Excessive progression in weekly running distance and running-related injuries (JOSPT 2014) (opens in a new tab): weekly progression
• Impellizzeri et al.: Acute:chronic workload ratio: conceptual issues and fundamental pitfalls (IJSPP 2020) (opens in a new tab): why ACWR is contested
• Abbiss & Laursen: Describing and understanding pacing strategies during athletic competition (Sports Med 2008) (opens in a new tab): pacing profiles
• Riegel: Athletic records and human endurance (American Scientist 69:285–290, 1981): the 1.06 power law
• Vickers & Vertosick: An empirical study of race times in recreational endurance runners (BMC Sports Sci Med Rehabil 2016) (opens in a new tab): where Riegel fails
• Jeukendrup: A step toward personalized sports nutrition: carbohydrate intake during exercise (Sports Med 2014) (opens in a new tab): carbohydrate g/h by duration
• Jeukendrup: Training the gut for athletes (Sports Med 2017) (opens in a new tab): gut training
• Thomas, Erdman & Burke: ACSM/AND/DC Nutrition and Athletic Performance (MSSE 2016) (opens in a new tab): race and pre-race carbohydrate
• Sawka et al.: ACSM position stand, exercise and fluid replacement (MSSE 2007) (opens in a new tab): fluid targets and sweat rate
• Hew-Butler et al.: 3rd International Exercise-Associated Hyponatremia Consensus (BJSM 2015) (opens in a new tab): drink to thirst
• Racinais et al.: Consensus recommendations on training and competing in the heat (Sports Med 2015) (opens in a new tab): acclimatisation, hydration, sodium
• NHS: Heat exhaustion and heatstroke (opens in a new tab): signs and first aid
• Wehrlin & Hallén: Linear decrease in VO₂max and performance with increasing altitude (Eur J Appl Physiol 2006) (opens in a new tab): altitude losses
• Chapman et al.: Defining the "dose" of altitude training (J Appl Physiol 2014) (opens in a new tab): best living altitude
• TrainingPeaks: Power training levels (Coggan) (opens in a new tab): cycling power zones
• TrainingPeaks: The FTP test, physiology and new protocols (opens in a new tab): the 20 min test and its 95% factor
• Heiderscheit et al.: Effects of step rate manipulation on joint mechanics during running (MSSE 2011) (opens in a new tab): cadence and joint loading
• Nigg et al.: Running shoes and running injuries: mythbusting (BJSM 2015) (opens in a new tab): comfort filter
• Hoogkamer et al.: A comparison of the energetic cost of running in marathon racing shoes (Sports Med 2018) (opens in a new tab): the 4% shoe
• Schumann et al.: Compatibility of concurrent aerobic and strength training (Sports Med 2022) (opens in a new tab): interference effect