Welcome Betty @hulst43, I've had one knee replaced and the other one is bone on bone but so far isn't causing me any pain although my right leg with the knee replacement seems to be a little stronger. I think the key to knee replacements is faithfully doing the pre-surgery and post-surgery exercises to strengthen the leg muscles. I'm also 80 and have difficulty walking due to lower back issues but I do ride my exercise bike 4 or 5 days a week for at least 30 to 45 minutes a day. I also have a stand up rowing machine that I use for 10 to 15 minutes a day to help with the leg muscles.

There is another discussion that might be helpful started by @10nsmom who had a bilateral knee replacement.
--- Total Knee Replacement: https://connect.mayoclinic.org/discussion/total-knee-replacement-2/

Have you discussed the knee replacements with your doctor or surgeon to see if they have some suggestions for you?