Why an extra day low on the mountain is the single best thing you can do for your summit day.
Altitude sickness is not a fitness problem. Strong trekkers get it, and it has very little to do with how fast you can climb.
As you gain height, the air holds less oxygen for the same volume of breath. Your body responds by breathing deeper, then over hours and days by changing blood chemistry. That second process is the one that keeps you safe, and it cannot be rushed.
This is why a route with an extra night at a middle camp is worth choosing over a shorter one. On the two Pangarchula routes, the seven-day option exists for exactly this reason. Walk high, sleep low, drink more water than feels necessary, and tell your trek leader early if you have a headache that does not clear.
