Endurance by Design doesn't invent training theory — it applies the methods coaches and exercise physiologists have used for decades, and does the bookkeeping for you.
Below is exactly how your plan is created, how it tracks your training, and how it adapts — with references to the research each part is grounded in. The short version: you tell us your goal race, its date, and how many hours a week you can train; from there the app works backward from race day to build a periodized plan and adapts it as you go.
A training plan only means something relative to a date. When you enter a goal race, the app anchors the entire schedule to that day and counts backward, so the hardest, most race-specific work lands in the final weeks and the taper finishes exactly on race morning. This is the backbone of classical periodization — organizing training into progressive phases that peak on demand rather than drifting forward from "today."8
What you provide shapes the plan:
The plan is divided into phases, each with a different job:
| Phase | Focus | What it builds |
|---|---|---|
| Base | Aerobic volume, mostly easy | The engine — durability and fat metabolism |
| Build | Threshold work on an aerobic base | Sustainable speed |
| Peak | Race-specific intensity and pacing | Sharpness for your event |
| Taper | Sharp volume cut, intensity kept | Freshness without losing fitness |
Within every phase, the intensity distribution follows the polarized 80/20 model: about 80% of training time at low intensity (Zone 1–2) and about 20% at high intensity (Zone 4–5), deliberately avoiding the "grey zone" of moderately-hard Zone 3 work that costs a lot of fatigue for modest benefit. Studies of how elite endurance athletes actually train — across running, cycling, rowing and cross-country skiing — repeatedly land near this distribution, and controlled trials show it outperforms threshold-heavy training for many athletes.1,2
Two guardrails keep progression safe:
Generic "easy / moderate / hard" labels aren't much use on the road. Where you've provided a recent race time or personal record, the app projects your training paces from it using the framework in Jack Daniels' running formula — the widely-used method that derives easy, marathon, threshold, interval and repetition paces from a single performance benchmark.7 Heart-rate zones are set as percentages of your maximum or threshold heart rate, and power zones (for cycling) as percentages of FTP, following standard zone definitions.4
Because these are projections, they're a starting point, not a verdict. As real workout data comes in, the numbers you actually hit inform how the app reads your training. Thresholds drift over a season, so it's worth updating them every few weeks during a serious block.
To adapt a plan intelligently, the app needs a common currency for "how much did that workout cost." That currency is a single Training Load score per workout, based on the exercise-physiology principle that training stress is a product of how long you went and how hard it was relative to your threshold:4
Training Load = duration (hours) × relative intensity² × 100where relative intensity is your average intensity for the session relative to threshold. One hour right at threshold scores 100. Because intensity is squared, hard work costs disproportionately more than easy work — which is precisely why 80/20 works: you can bank large amounts of low-cost aerobic volume and spend your limited recovery budget on a few genuinely hard sessions. When a workout comes in with heart-rate data, the app computes the load from your average heart rate relative to your lactate-threshold heart rate, with sensible clamps so a stale threshold can't produce a nonsense score.
Every day's load then feeds three rolling averages — the fitness–fatigue (impulse-response) model first described by Banister and now standard across training platforms:3,4
| Metric | What it is | Window |
|---|---|---|
| Fitness | Chronic training load — what you've banked | ~42 days |
| Fatigue | Acute training load — what you've done lately | ~7 days |
| Form | Fitness minus Fatigue — your freshness | the difference |
The paradox baked into this model is that the work which raises Fitness also raises Fatigue — so getting fitter makes you feel worse in the short term. Form only appears when you ease off and let Fatigue fall faster than Fitness. That is all a taper is.
The taper is where races are won or lost, and the right taper depends on the event. A tapering meta-analysis found the most reliable performance gains come from cutting training volume substantially (roughly 40–60%) over one to three weeks while maintaining intensity and keeping training frequency high.5,6 The app applies a distance-specific taper built on those principles:
| Race | Taper length | Approach |
|---|---|---|
| 5K / 10K | 4–6 days | Short, sharp volume cut; a few strides the day before |
| Half marathon | 7–10 days | ~45% volume cut, intensity retained |
| Marathon | 2–3 weeks | Graduated cut, longest run pulled back progressively |
| 70.3 | 10–14 days | Progressive cut with a capped final few days |
| IRONMAN | 2–3 weeks | Long, graduated cut into race week |
After the race, the plan schedules distance-appropriate recovery before any hard work returns — a couple of easy days after a 5K, up to a two- to three-week rebuild after an IRONMAN — starting with full rest. Your B and C races are handled proportionally: a B race gets a short mini-taper (a few lighter days before, a couple of recovery days after) so you arrive fresh without derailing your A-race build; a C race is treated as a hard training day with a single easy day to follow.
A plan that can't bend is a plan you abandon in week three. Endurance by Design adjusts on three timescales:
Daily — readiness. Each morning the app blends recovery signals into a single readiness score, weighting sleep (25%), heart-rate variability against your own baseline (25%), recent fatigue (20%), stress (15%) and training load (15%). HRV- and readiness-guided training has growing support as a way to time hard sessions for when your body can actually absorb them.10 A low score suggests an easier day; a high score means you're clear to push. You're always in control — the score is guidance, not a lock.
Per workout — Smart Alternatives. Don't like today's session, or short on time? The app offers easier, similar, harder and time-crunched versions of the workout, each showing its training load and intensity so you understand the trade-off before you swap.
Across weeks — realignment and re-entry. Miss an easy session and it's quietly absorbed. Miss a key session and, if you're not close to your race, it's re-timed into the coming days — never stacking hard efforts on back-to-back days. Take a planned break or come back from illness, and the app eases you in with a gradual return ramp rather than dropping you straight back onto your old load — the same 10%-per-week logic that governs building up in the first place.9
The intelligence in Endurance by Design sits on top of this framework, not in place of it. The established methods above — the phase structure, the 80/20 distribution, the training-load math, the taper and recovery lengths, the ramp caps — are fixed, deterministic rules. The AI's role is to translate them into a specific, coherent, day-by-day plan for your race, your calendar and your hours, and to keep that plan sensible as your circumstances change. Every AI-generated plan is validated against those fixed guardrails before you ever see it, and if anything falls outside them the app falls back to a rule-based plan built from the same principles. The science decides what good training looks like; the AI handles the personalization and the bookkeeping.
Endurance by Design is a training tool for healthy adults. Its plans, paces, training-load numbers and readiness scores are estimates generated by algorithms and models — even the best predictive formulas are approximations of a complex, individual body, and they can be wrong for you.
The app does not provide medical advice, diagnosis or treatment, and it is not a substitute for a physician, a registered dietitian, a physical therapist, or a qualified coach. It does not detect or treat any medical condition. Endurance exercise carries inherent risks. Consult a physician before beginning or significantly changing any exercise program, especially if you are pregnant, have a heart condition, an injury, or any other health concern. Stop and seek medical attention if you experience chest pain, dizziness, unusual shortness of breath, or other warning signs during exercise. Use of the app is at your own discretion and risk; you are responsible for how you train.
Endurance by Design is an independent product and is not affiliated with, endorsed by, or sponsored by any of the authors or publishers cited above. References are provided to show the established methods our plans are built on.
Tell us your goal race and weekly hours, and Endurance by Design builds a periodized 80/20 plan tapered to race day, then adapts it every day. Free to download on the App Store, with a 7-day free trial.
