We weren't on a tour, but introducing some family members to France we did the following.....
We spent afew days in Paris first (no tour, so we visited some things that you may see on the tour). Picked up a car at Orly and drove to Giverny in the a.m., visited Monet's house and gardens, ate lunch at the cafe there, then drove on to Rouen. You could stay 2 nights there (charming medieval town). Next day drove by way of the Route Des Abbayes and partway along the Seine River to Bayeux. You could spend 3-4 nights there (we spent just 1 night; some spend a week there, but that is more WWII history than I want in one time). Bayeux has a great church and museum, plus a nice area to walk around. With a car you can choose from a number of sights. Also cider routes and some good eats/drinks (camembert, calvados and shellfish.) We left the American cemetery after the flag-lowering ceremony and drove to Villedieu-les-Poeles (long-time bell-making.lace-making/copper foundry town) to spend the night (another chambre d'hote).
We went to Mont-St-Michel fairly early next day, going up the back way (steeper but less crowded) to the Abbey, and coming down through the more crowded street on our way out. Then we drove on to Dinan in Bretagne, where we stayed overnight. The next three days we did some exploring of Bretagne. Our family members had their own car, so went off on their own. We had some French relatives to visit, so met the others two days later at Quimper for aperitifs near the cathedral.
From Quimper we drove to our next stay, a chambre d'hote in Pluneret, near Auray, a cool little harbor area and good restaurants for Breton (savory) crepes and more shellfish. The following morning we went to Carnac to see the dolmens, and were off to Saumur at the western end of the Loire area. That was our longest drive (about 4 hours). All the others were fairly short between stops (per our plans). This was an introduction to the northern half of France for the family members, so a three-week trip covered a lot.
To continue, we stayed in Saumur, doing some wine-tasting. Next day we worked our way east...Chenonceaux in the morning, picnicikng on the grassy area (with tables) near the parking lot, then to Chambord in the afternoon. We stayed at another chambre d'hote near Chambord. (Very nice! Dinner with the owners from aperitif to digestif, lots of good conversation, and very reasonable at 25 euros pp, including wine). Nice lodging as well, with a terrific breakfast! Next day we headed south toward Burgundy. You would probably head back to Paris from here.
Of course, that was what we did in 2017, with 5 adults (3 newbies) and 2 cars. Most drives were fairly short, usually 1 to 1 1/2 hours. Only Pluneret to Saumur (via Carnac) was 4 hours. Suamur to Chambord was 2 1/2 hours if we drove straight through, but we stopped to visit Chenonceaux on the way, so not a long drive on either side of Chenonceaux.
I hope this itinerary gives you some ideas. I used Viamichelin.com for finding routes. They often give you options of fastest, cheapest, most scenic, etc. I always add time to what they say to allow for gas/food/comfort stops and the odd traffic problem. We also try to take the smaller D roads. The autoroutes are fast, but less interesting to me. And they may have tolls. (Except in Bretagne. No tolls allowed there.) When booking hotels, I tried to get ones with parking where possible, even if there was a (reasonable) charge. Sometimes a parking spot is hard to find. We did this drive in early May, so you will be there when we were. We didn't run into too many crowded places.
If you want to skip Paris before your road trip, I would maybe take a train to Giverny arrival day and spend the night. Then train to Rouen next day and pick up a rental car on your way out of town. And you're off!
We are also 80 somethings, but did this trip in our mid 70s.
Bon voyage! Amusez-vous bien!
