I should add that you must go early in September otherwise the lifts, navettes, Montenvers railway will be closed, and they are essential to get the best out of Chamonix walking. It is in a typical glacial valley, U shaped, very steep initial slope so you need mechanical help to get up on to the more open upper slopes, or spend all morning slogging up a path. Most amenities close down in the middle of the month to prepare for the ski season.