Not much of a chance of a repetitive stress injury in cycling, unless you are mashing big gears all the time, knees will pay the price with that. If the bicycle is fitted properly, by a shop or by you, it is very safe exercise. Lots of 50+ riders are in the sport because of this aspect, we really hate to hurt and go to great lengths to avoid pain. Pain hurts (deep and thoughtful statement for Capt. Obvious fans) If you want to mix in other training disciplines there are many that use weights, yoga, some runners and swimmers here, too.

Personally, I can't use running because of spinal surgeries and cycling offers me a fairly safe way to get aerobic exercise. Best wishes in your search and welcome to the 50+/bicycling.

Bill