Entered once. Copied onto each plan when you save, so changing your weight later won't rewrite plans you already made.
Sweat rate is the widest-variance number in the model and the one worth measuring: weigh yourself before and after a one-hour ride, add back what you drank. Every litre lost is about 1 kg.
Drop in a GPX, or paste a route link, and distance and climbing get read from it.