Short answer
Fit enough to walk five to seven hours a day, on consecutive days, on uneven ground. That is a moderate rather than an athletic standard. Everest Base Camp turns people back through altitude, not fitness — the trail tops out at 5,545 m on Kala Patthar, and no amount of training prevents altitude sickness.
The useful benchmark is this: can you walk for six hours with a daypack, up and down a few hundred metres of hill, and get up and do it again the next morning? If yes, you have the fitness. Marathon runners and cyclists are not noticeably better off, because the limiting factor is oxygen availability, not cardiovascular capacity at sea level.
What training does buy you is margin. Twelve weeks of hill walking with a loaded pack, twice a week and building to five hours, means the walking is comfortable enough that you notice the altitude early rather than mistaking its symptoms for fatigue. That distinction matters, because the first response to altitude sickness is to stop climbing.
Age is a poor predictor. We have taken travellers in their seventies to Base Camp and turned back people in their twenties. The reliable predictors are acclimatisation discipline, hydration and honesty about how you feel at Dingboche.
| Measure | Typical | Hardest day |
|---|---|---|
| Walking hours | 5–7 hours | 8–9 hours, Gorak Shep to Base Camp and back |
| Daily ascent | 400–600 m | About 800 m, Phakding to Namche |
| Pack weight carried | 5–7 kg daypack | Same — porters carry the duffel |
| Sleeping altitude | 3,440–5,164 m | 5,164 m at Gorak Shep |
| Consecutive walking days | 12 of 14 | No rest day after Dingboche |