Has anyone used the services of France Just For You to organize a solo holiday?

I don't know anything about that company. However, I believe if you search the web under terms like disable travel France or wheelchair travel France you will find companies that specialize In helping people with mobility challenges. They might be a reasonable place to start planning, even though you can walk. I believe that some of them note they also help "slow walkers".

I would just like to add how excited and in awe I am of you.

I haven't yet hit the mobility challenges, so I am going to learn with the answers you get rather than give advice. It's a question I haven't had a reason to think before and now will file away for the future. Thank you for asking the question, I think what you're doing is awesome, and I now challenge myself to make a similar trip when age and mobility are an issue. To travel solo is a marvelous thing!

I had an immediate reply from Emilie at France Just For You that unfortunately they plan only self-drive itineraries. They have good reviews and would be worth investigating if that is what you are after.
Following Sherry's suggestion, I searched for 'slow walking France' which led me to a couple of companies I didn't know about - La Belle France and The Enlightened Traveller; the latter in particular has several 'light' options with distances closer to what I can manage (although none in the Dordogne), so I shall be looking into those in more detail.

To Nancy: I really became aware of my limitations in Provence in June/July this year when it was very hot. Fortunately, the InnTravel holiday I was on allowed me to short-cut the lengthier hotel-to-hotel routes by using the taxi that was transferring my luggage, but this is not always possible. In France, I do prefer to walk and explore on my own (I do have reasonable French) but in other countries I'd always go with a group. It is good to alternate between these. Even for solo walking, booking though a company has many advantages in that accommodation and luggage transfers are assured and already paid for.

Just turned 70 last month and I'm fortunate not to have to deal with what you described, and I admire your spirit for wanting to continue to travel in Europe. Have you considered traveling with someone who is interested in the areas that you mentioned and might also traveling solo? They could drive and you could share some of the expenses (car, gas, etc.) and otherwise go your own way as you like. You may even find that you have things in common that you could enjoy together. It seems to me from reading your posting and others you've responded to that you're an experienced traveler who just needs a bit of help getting around from place to place, but otherwise do not have walking or other limitations. On the forum there is a section where you can search for travel partners. I don't know what is on there right now, but I believe it is worth your looking into. You did not say if you are you a man or a woman and, although it wouldn't matter to me, it could help anyone you in Travel Partners to decide if they would like to join you. I would liken this idea to be like Rick Steves My Way tours but for a solo traveler. Since 2007 I have taken my wife and her now 91 year old mother on three trips to Europe, first Italy, then Scotland and England and lastly to France where we spent one week in Normandy and one in Provence. Last year and again next year it is Hawaii for the three of us. Having been the chauffeur on all of our trips (as well as planner, organizer, financier, pack mule and tour guide) I'm well aware that having a car gets you to off the beaten places not available through public transportation. Hope you are able to make this trip. Both of the places are on my travel wish list too. Good luck.

The Dordogne is not an area that lends itself to public transport travel. We are also in our 70s and travel independently but it gets tougher -- we probably have one or two more driving trips and hope to get to the Dordogne and Tuscany again before it is too much. We are sitting in an apartment in St. Petersburg right now and are managing fine, but we were very happy that we hired a guide to drive us to Pushkin for the sights there rather than trying to put it together ourselves. (we have done everything else including 3 days at the Hermitage on our own)

I am not much for tours, but I think if you can't drive and want to see places like the Dordogne, a tour is the way to go. Rural areas of France and Italy don't have public transport well suited to tourists. Bus schedules are designed for school kids and workers and hill towns and villages are rarely accessed by train. The other option is to stay put in a small town and just enjoy being there for a week, perhaps arranging local drivers for day trips.

Thanks, Jim. I really do prefer to walk on my own in France, and have done so with great enjoyment and without problems for the last 16 years. It's just that the distances in the inn-to-inn itineraries offered for most 'self-guided' holidays are now mostly a little too far or strenuous for me. This time I've decided to try one of the Enlightened Traveller's 'light' routes (not in Dordogne but in an area of Provence I don't know at all), and end with a Headwater 'easy' walk in Burgundy, with an extra day or two in Vézelay. In between - as Jane suggests - a few days staying put in Sarlat, which is accessible by train and has plenty of walking options in the vicinity. I can always hire a bike to go a little further afield from there, if I wish. I have already explored many parts of the Dordogne on previous self-guided walking and cycling holidays, and there are a couple of places in particular that I'd like to revisit from Sarlat.
Best wishes for the rest of your stay in St Petersburg, Jane.