The single biggest reason people fail to reach Everest Base Camp isn't fitness — it's going up too fast. Altitude doesn't care how many marathons you have run, so the most important part of your plan is the pace.
The rule of thumb above about 3,000 m is to raise the height you sleep at by no more than roughly 300–500 m a day, and to take a rest day every few days. On the EBC route that means a day at Namche Bazaar and another at Dingboche, with a third in reserve if anyone in the group is struggling. “Climb high, sleep low”: on rest days, hike a few hundred metres higher, then come back down to sleep.
Know the signs of acute mountain sickness — headache, nausea, dizziness, poor sleep and loss of appetite. Mild symptoms mean stop and rest; worsening symptoms mean go down. Confusion, stumbling or breathlessness at rest are emergencies: descend immediately and get help.
Drink plenty of water, eat well (dal bhat helps), go easy on alcohol, and talk to a doctor before your trip about medication such as acetazolamide. Above all, give yourself spare days so you never feel pressured to keep climbing when your body says no.
