The GR70 — the Chemin de Stevenson — runs from Le Monastier-sur-Gazeille in Haute-Loire down through the Cévennes to Saint-Jean-du-Gard, broadly following the route Stevenson walked. Florac is roughly halfway and the standard overnight stop for through-walkers. The whole trail takes about ten days; a shorter stretch from Le Pont-de-Montvert to Florac (two days) gives a good taste of the wilder eastern section.
Several outfits in Le Monastier and Florac will rent you a donkey for the day, the week, or the whole route.