Public transport from La Rochelle to Ouisterham

Am really keen to do the Velo Francette next May.  Am already thinking about how you get back from La Rochelle to Ouisterham without going via Paris - any suggestions?  Also my husband and I are not in the first flush of youth and don't know if we will manage more than 30 miles a day so am wondering if it is essential that you pre-book accommodation or whether you can get it on an ad hoc basis?