"Any suggestions for this sad sack one time traveler?"
Well, first, stop thinking of or referring to yourself as a "sad sack" and "one time traveler," or that is what you will be! On the other hand, if you think of yourself as someone who can still travel but needs to do so with modifications, you'll start with a much better attitude. Also a more realistic one - plenty of people who can only walk a short distance do all kinds of traveling.
Do start with Rick's old book. Since it's free, you can see some of the options for getting around London, Paris, etc.
Do look into assistive devices like canes, wheelchairs, rollators, hiking poles etc. What you use in your everyday life may or may not work for travel.
Plan to take more taxis and less public transit than you did in the past. Always have addresses written down, so you can just show them to a driver to avoid misunderstandings, and keep taxi numbers handy, so you can always call one if you need one. See if Uber/Lyft/Gett are available in places you are going.
Many museums have loaner wheelchairs available (this came in very handy in Dresden when my mother injured her knee getting off the train). Museums are some of the most tiring places, as you're doing a lot of standing with a little walking in between; it can be worse than normal walking, and it can be hard to find seating.
Bathrooms can be a challenge when out and about if you have any difficulty with stairs. It's very common for a ground floor restaurant to have bathrooms up or down a flight of stairs, with no elevator.
I'll emphasize the point that in Europe, the first floor is actually one floor above the ground floor; so, if you need to make sure you are not going to have to climb a flight of stairs, make sure you can get a ground floor room - or book a hotel with an elevator and don't take chances.
Does a cruise appeal to you? They are very well set up for travelers with all kinds of disabilities. If you want some inspiration, even if you're not cruising, look at the Disabled Forum on Cruise Critic. https://boards.cruisecritic.com/forumdisplay.php?f=190 Once you start reading about people who need tube feedings (can't eat at all by mouth), those confined to wheelchairs, etc, you'll see that by being able to walk a bit, you're "ahead of the game."
I could have sworn that there was a recent report of someone who traveled with a family member with some mobility problems, and how they were able to work around the challenges and have a great trip. I can't find it now, but if I do, I'll post the link.