Your highest elevations will be the first three days from Geneva, and then the two weeks starting 3 days before Le Puy. I would anticipate snow in these sections, which may obscure the trail. That is, in the sections where it actually is trail. But two-thirds of the route (after Le Puy at least) is farm tracks and back roads.
This being the low season, the local tourism offices should have more time available to help with calling ahead to make bookings. Also your innkeeper today would be a wonderful resource for calling ahead to make tomorrow night's booking; they're generally quite aware of who is open, (or says they're open but actually off visiting children elsewhere etc) in their vicinity. Once you get to Le Puy, MMD picks up coverage for lodgings, but again just because an establishment plans to be open doesn't necessarily translate into actuality. Also, many establishments have Web listings these days, so you can possibly make reservations by email.
Getting supplies may be an issue. In the smaller towns, bakeries and markets may be closed seasonally -- the smallest places are already losing these businesses altogether. So you may want to (1) carry more supplies - 3 to 4 days rather than 1 or 2 days' worth, and (2) check with your next night's host as to supply availability when you call for booking. With your youth and fitness, the extra weight necessary shouldn't be an issue for you.
If you can handle field exercises in the Finnish winter, you can surely handle the Aubrac plateau. Bon chemin and keep us posted on your experience!