Not every race is 5K, 10K, half, or full
Local races rarely respect the standard menu. A 5.5-mile trail race, a four-mile turkey trot, a 15K, a 10-miler, a 12K charity run — these are the races people actually sign up for. Step 1 of the setup wizard offers the four standard distances plus Custom, where you type the exact length of your race in your plan's unit.
Supported range is 1 km to 100 km (about 0.6 to 62 miles), entered to one decimal place.
Trained like the nearest distance, raced at yours
A training plan is not really shaped by the race's exact length — it's shaped by the kind of race it is. A 10K plan spends its quality sessions on intervals and threshold work and peaks with a long run around 14 km; a marathon plan spends them on marathon-pace work and peaks around 32 km. Those differences are large and structural. The gap between 8.85 km and 10 km is not.
So Pheidi splits the two ideas apart:
- The archetype — the nearest standard distance to what you entered. It drives everything that's keyed to race type: peak long run, which workout types the quality pool draws from in each phase, the mid-week long-run share, the pace tables, and the recommended plan length.
- The raced length — the exact number you typed. It's what your plan is named after, what race day shows, what the race-pace math uses, and what every export and share card says.
Enter 5.5 mi and the wizard tells you exactly what it's doing: "Trained like a 10K, raced over 5.5 mi." The archetype is never hidden — it's on screen while you choose, and again in the plan settings blade if you change distance later.
Which archetype your distance maps to
Pheidi picks the standard distance closest to yours in kilometers. Ties go to the shorter one.
| Your race length | Archetype | Default plan length |
|---|---|---|
| Up to and including 7.5 km (4.7 mi) | 5K | 10 weeks (8–12) |
| Over 7.5 km up to 15.5 km (4.7–9.7 mi) | 10K | 12 weeks (10–14) |
| Over 15.5 km up to 31.6 km (9.7–19.7 mi) | Half Marathon | 14 weeks (12–16) |
| Over 31.6 km (19.7 mi) | Full Marathon | 16 weeks (16–30) |
The plan-length warnings on the race-date field follow the archetype too: pick a 15K eight weeks out and you'll get the 10K minimum-weeks warning, because that's the block the plan is actually built from.
Beyond the marathon, the archetype stays Full Marathon — a 50 km plan is built with marathon structure and raced over 50 km. Pheidi does not yet do ultra-specific periodization (back-to-back long runs, time-on-feet targets, vert).
Predictions and pace at an odd distance
VDOT tables only exist for standard distances, so a finish time over 8.85 km can't be looked up directly. Pheidi converts it first using Riegel's formula (T₂ = T₁ × (D₂/D₁)1.06), the same relationship used for cross-distance race predictions, then reads the archetype's table. A 45:00 over 5.5 miles is a slower effort than a 45:00 over 10K, and the implied fitness comes out correspondingly lower — the odd distance never flatters or penalizes you by accident.
Race predictions work the same way in reverse: your predicted finish is computed at your exact length, not at the archetype's. The training-pace estimate also interpolates between the standard anchors, so a race sitting between 5K and 10K gets a race-pace factor between the two rather than snapping to one.
Units and your custom distance
The number you type is stored in the unit it was typed in — see how plan units work for why Pheidi never converts stored plan values. In the wizard the input carries a mi or km suffix matching your unit selection. The toggle changes how your plan is written, not which race you're running, so switching it converts the length you entered: a 5.5 mi race becomes 8.85 km, still the same race and still built on the same 10K structure.
Changing it later
The plan settings blade has the same Distance dropdown with a Custom… option. Changing your race length changes the shape of the block, so saving it regenerates the plan — the old one is paused, not deleted, and your logged history stays with it.
Where the length shows up
- Plan name: auto-named plans read "5.5 mi – Dec 2026".
- Calendar: the race-day cell shows your exact length.
- Exports and sharing: the calendar export, shared plan page, printable plan, and share cards all use it.
- API: the public plan API reports
distanceLabelandraceLengthalongside thedistancearchetype, so a consumer can see both what you're racing and what structure it was built from.
Custom race distances live inside the Pheidi app — sign in to build a plan for your exact race.
See also
- How Onboarding Works The six setup steps Pheidi uses to build your personalized training plan, race goal and units, experience, volume, pace mode, schedule, and generation.
- How Plan Units Work Plans are built natively in miles or kilometers, and you can rebuild yours in the other unit any time.
- How Plan Generation Works How Pheidi builds your personalized training plan: periodization, phase splits, shape-based load distribution, taper math, and base building.