I will also recommend Rick Steves Rome guidebook along with his Italy one as well.
Walking tours are really the way to go. Rick will have recommendations. I don't remember if we used one of his recommended or a different operator when we visited Rome for several days. The operator we had offered many different tours, and there were 4 that we wanted to do and fit into our schedule, but the fourth one was either free or at least half off - so bonus. Every guide we had was awesome.
Search online, check out TripAdvisor for reviews and pick an operator that meets your needs and budget.
ETA - sorry I missed where you have some issues walking, however, still search and read reviews of tour operators.