A 50K is 4.9 miles longer than a marathon, but the difference between the two is bigger than five miles. The terrain is harder, the time on feet is longer, and the mistakes — fueling, pacing, foot care — start mattering. Below: a 16-week plan from a marathon-fit base to a 50K finish.
Who this plan is for
- You've finished a marathon in the last 18 months
- You're running 25–35 miles per week comfortably
- You can run a 90-minute long run without crashing
- You have 16 weeks of consistent training time
If you don't have any of those, build a marathon base first — start with the Hal Higdon Intermediate or similar — then come back to this plan.
The structure: 16 weeks, three phases
Weeks 1–6 (Base): 30 → 38 mpw. Long run grows from 12 to 18 mi. Most miles easy.
Weeks 7–13 (Build): 38 → 48 mpw. Long runs 18–22 mi. Introduce back-to-backs.
Weeks 14–16 (Taper): 48 → 30 → 20 mpw. Last hard effort week 14.
A sample mid-build week (week 11)
- Monday — rest or 30 min easy + mobility
- Tuesday — 6 mi easy with strides
- Wednesday — 8 mi with 4 × 4-min uphill repeats
- Thursday — 6 mi easy
- Friday — rest
- Saturday — 20 mi long, on terrain similar to your race
- Sunday — 8 mi easy back-to-back, very slow
Total: 48 mi.
The back-to-back is the new workout
The marathon you trained for didn't ask you to run on tired legs. The 50K does — the last 5 miles of a 50K feel like the last 10 of a marathon. The back-to-back long run on Sunday teaches your body that running on Saturday's residual fatigue is a thing it can do. This is the single most important workout you'll add to your training.
Sunday's pace should be embarrassingly slow — 60–90 seconds per mile slower than Saturday. The point isn't speed; it's adaptation.
Vert
Most 50Ks have meaningful elevation. If your race has 4,000+ ft of climb, your training needs at least 1,500–3,000 ft of vert per week in the build phase. If you live somewhere flat, treadmill incline at 8–10% for 30 minutes once a week roughly substitutes.
Fueling — start practicing now
A 50K takes 5–7 hours for most first-timers. You'll burn 2,500–4,000 calories. Plan to consume 200–300 calories per hour during the race. Train this on every long run from week 7 onward — same gels, same flask mix, same intervals.
Most first-time 50K runners under-fuel because they think "it's just a long run." It isn't. The race lasts twice as long as a marathon and your stomach is still digesting at hour 4. Feed it deliberately.
Race-week routine
- Last hard workout: Tuesday of race week — 5 × 2-min at threshold
- Wednesday: 4 mi easy
- Thursday: rest
- Friday: 2 mi shake-out + strides
- Saturday: rest, hydrate, eat carbs
- Sunday: race
Race-day pacing
Run the first 25% slower than feels right. The 50K rewards conservative starts more than any marathon distance. Most first-time 50K finishers describe the same arc: feel great for 15 miles, struggle from 18 to 25, find a second wind in the last 6.
Walk every climb in the back half. Eat at every aid station. Don't try to "run through" any low patch — when energy drops, walk for 5 min, eat 200 calories, drink electrolytes, then resume running.
What to drop when life gets in the way
- Tuesday strides — easy to skip, easy to recover from
- Wednesday hill repeats — important but recoverable if missed once
- Thursday easy run — pure recovery
- Saturday long run — non-negotiable
- Sunday back-to-back — non-negotiable
What's next after the 50K
Most 50K finishers either swear off ultras forever or sign up for a 50-miler within 3 months. There's almost no middle ground. If you're in the second camp — that's the next guide. The marathon-to-50K transition is the hardest one you'll do; everything after is incremental.