Like others have said, I wouldn't worry about wearing out the pads, that's what they're designed for. Blowing out your tire on long downhills if you ride the brakes is a VERY real threat. Last week on the descent from Mont Ventoux I passed 4 cyclists with blowouts, at least 2 of them were later confirmed to be from overheated rims.

I imagine the heavier you are the more heat you will generate in order to maintain a given speed.