AliceTara said:(i) how easy it is to rock up in the nearest town and find accommodation (e.g. are there loads of youth hostels or something similar)
Youth hostels, no. But you should be able to find a chambre d'hôtes (B& or some such. Depends how late you are into autumn but camp sites should still be open... and pretty quiet. Depends where you though.
(ii) how easy it is to take your bike on a train in France without pre-booking?
Regional trains, I'd say negligible problem... but you could strike unlucky. TGV is a different issue. If it was a vital journey, I'd pre-book it. That said, you could strike lucky!