One hidden advantage in taking the metro is that you get a lot of exercise from walking up and down those metro stairs. I got my legs in shape during my ten days in Paris a year and a half ago.
Regarding the other parts of your questions: Are your plans set in stone? I visited Dijon and Lyon two years ago and the impression I got was that while Dijon was a nice place I got bored of it after a few days. Consequently, I wished I had budgeted more time for Lyon. I think I spent like 6 days in Dijon and three in Lyon. There is also a metro system for Lyon. I visited the major church there--Notre Dame (in Lyon)--during Christmas time and had a delightful conversation with a church guide there. She was trying to explain what a creche scene was in her limited english while I was trying to communicate with my very limited French speaking skills.
Whatever you choose enjoy your trip!
FWIW - slowing down and enjoying is one of the charms of Paris. Robyn
Agreed, I have mobility problems too but I walked alot. I remember taking a walk from Pont Neuf after a meal and just exploring via foot. A while later--I lost track of time--I came upon the Pantheon. This was interesting because I am somewhat directionally incompetent--I get lost easily--and I had been trying to figure out which metro station would be the closest to get there (Pantheon). I'm still not sure how I got there as I thought I was going in the opposite direction toward the right bank. Bummer was that the top of the dome was closed.
Another anecdote is that after half a day at the Lourve I started to walk in search of a restaurant to eat lunch. I walked for a while and came upon this place in the fourth. Since I was extremely hungry, I just stayed there and ate lunch. Luckily they had roast pigeon it was a great meal and I ended up eating there multiple times. On my last night there they gave me a complimentary glass of red wine and free hors d'oeurves.
The only thing that kind of irked me was that movies take forever to start there. I wish I had known that the time posted is not the actual start time. The previews and whatever else took around 45 minutes.