@margomorrison , may I suggest that if you're unsure, then just add a day or two to your plans ( assuming that's possible) .
Just because most walk it in 4 to 5 days doesn't mean you HAVE to. That's just sticking to the so called 'Stages'. There are many little towns along the camino and whilst not all have Albergues many do, or at least a small Hotel etc.
Lots of people have posted here about taking it slowly, one or two planned 8 day's (for health reasons). If you've been training, even allowing one day more will make a big difference to the mileage you need to cover on a daily basis.
Sorry, appreciate that's not really a direct answer to your transport question. Worst case taxis are everywhere, and I believe one or two towns have buses between but sorry I can't recall which.
Buen Camino!