Oo, long, Dinan is in northern Brittany, Carnac in southern Brittany.
Google maps say 2 h 6 min drive.
Northern and southern Brittany are far apart and belong to two distance bases.
If you base yourself in the north, in, say, Dinan, you can visit St Malo, St Briac sur Mer, Cancale, Erquy, Mont St Michel, etc.
If you base yourself in the south, you can see Carnac and several beautiful islands near Vanne. I don't know the area at all but have always wanted to go.
But one can't use one base for both areas. The distances are too great. Maybe one week per base? Or at least 5 days per, if you don't want too much run-around.
Back to Dinan's b&b. The street outside is a small street but is not narrow (not as narrow as many Paris sidewalks!) and not that hilly (but having lived in two famously hilly cities in the world, I may be the wrong person to address this question).
As for how you walk: if you were like the 1st time I met you, I'd say don't stay there. But you walked much better the last time I saw you; you just could not walk for too long. So I would tend to think that since you can navigate the narrow sidewalks of Paris and the 1st flr walkup in the 9th, you will be ok.
Whatever you choose will be a great choice.
"And time yet for a thousand indecisions"…
