I’ve got a flight credit I need to use either by end of April or end of 2023, depending on whom I talk to, so I"m leaning towards end of April to be safe. But, I also have a physical issue now where sitting for long periods of time is ill advised. I have a cushion I am supposed to sit on which causes me to start having other problems if I sit on it too long; for example, when driving I need to stop and walk around if it’s been close to an hour. But this prevents the chronic pain so I deal with it. I asked my dr about this and said that to use my flight credit I’d need to book rather long distance flights, and even with breaking up the trip into segments I’m still looking at flights of at least 4 hrs. He said he can give me a letter, and he already has a form letter he’s used for other patients, which I’d give to the flight attendant that says I have pain when sitting and need to be able to walk around. He stressed that I need to follow the rules of course, so need to remain seated while ordered to, but that otherwise I should be “up and walking around all the time.” That seems…impossible! I mean, where do you go except up and down the aisle and into the bathroom? I see wha the means but I also feel uncomfortable with the idea of walking around a plane. And, I tend to like the window seat, but that would be a terrible idea in this instance. I am lucky in that I have enough funds in the credit to get a first class or business class seat on all my flights so I’d just get the aisle seat of one of those. It’s also weird b/c this would be solo so the person next to me isn’t in on it. I’d appreciate any advice. I am not going to let this stop me from flying and going where I want to go, but it is awkward.
I did not end up actually flying in this instance but I can just tell you what my doctor told me.
I had just had back surgery and won a trip to New York to see a celebrity and when I told my doctor I really wanted to go he said it was ok but I had to tell the flight crew my situation. (You really aren’t supposed to sit much after back surgery.) He assured me it would be okay and said if I was hassled I should threaten to show them the surgery scar. 
So I’m not much help here but my doctor seemed to feel it was pretty common if that helps.
I forgot to add that if you are in business or first it’s usually not that inconvenient to exit the row especially with some of the configurations. There are planes where every seat in that class has it’s own “exit area.” And business travelers are often the people in premium classes and are very used to sitting next to solo flyers.
Thanks. One of the difficulty things is that I don’t appear as though I have a problem.
I think people are getting better at understanding invisible disabilities, and you don’t need to apologise anyway. I don’t have any issues but would get up at least once an hour, do some heel lifts and walk down the aisle and back.
I’d ask the flight attendant if you can walk and stretch in the galley, if they are not actively serving - in business of first, I’m sure they would agree.
@Kitty_Ellas_Mom , one more thought is some airports now have a way to flag invisible disabilities - normally a sunflower pin or lanyard, which flags to staff that you may need extra consideration. If you ask at information at your departure point they may be able to help you.