I'm 67, both knees are bad, both hips are bad and have osteopenia and I always walk in the parks. Heck, I walk between Epcot and HS most of the time or take the boat halfway. I find lots of opportunities to sit down and rest, go back to the resort mid-day to take a break. At night if there is a bath tub in my room I take a good long hot soak, if there isn't I try to get in the hot tub at the pool mid-day. I always travel with a good old fashioned heating pad that goes on my back for a while at night and advil/aleve are my friends. I take two Aleve in the morning before I leave the resort and usually, two Aleve and an Advil before I go to bed and carry a small pill box into the parks in case I need a boaster. Good shoes help and I do always have knee braces with me although I've yet to have to wear them into the parks. Also a huge fan of some sort of pain relieving cream. It's completely doable if you listen to your body and rest when needed. Oh, I've never bought any kind of skip the line pass, I just stand in lines if I want to ride something. If I think it might be too painful at that time to do so, I just skip that ride and carry on.
Funny story - Hubby and I were at WDW early this December. I'm 13 years older than him. He is not a fan of WDW and this is only his second time going as an adult, I usually either go solo or with my son. While he is younger than me, hubby isn't in great shape, way over weight. So we are walking around Epcot and have walked from the Living Seas over to Mexico and he starts complaining his hips are hurting and he doesn't know why, it's never happened before. We went to AK the day before and we walked from the front of the park back to safari and back to the front of the park (stopping at Nomad Lounge for lunch) and that is the only walking he did that day. We grab lunch at Mexico and start to walk around WS and he has to stop about every 2 min. to rest his hips. He decided it was his style of walking so adjusted that. It didn't help. We got as far as Morocco and he just couldn't any more. I had him sit on a bench while I fast walked to France to get an ice cream and we took the boat across the lake back to the front of the park and back to the Fort (we were in our RV) and that was it for him. I know the issue was #1 he gets no exercise at home, works from home, has a riding lawn mower so basically moves from his recliner to the basement every now and again to use the computer there. #2 he is at least 60 pounds over weight so walking was putting too much pressure on his hips. So my old, worn down, arthritic body was doing better than his. I do walk every day, or at least every other day while at home for at least a mile or two, try to get up and move around the house about every hour and if anything is needed upstairs or downstairs, I'm your gal. I'm really think it's because I do try to move around a lot and lub up those joints every day that I was able to get around a lot better than him. Don't get me wrong, there are days when I'm sitting on the couch with a heating pad on my lower back/hips and that is all I'm doing that day because it hurts but most days, I've found if you move around at home, it's going to help you when you go to WDW.
Know your limits and rest, rest, rest a lot when at WDW.