240 miles of mostly off-trail sounds like you're doing pretty well! I agree that condition is more important than age. The kicker is that getting in shape takes more time and care as we get older. Especially with regard to joints. The combination of shortened range of movement and loss of elasticity leads to unexpected pulls. And it takes a long time to stretch out the offending tendons without re-injuring yourself.

One piece of inexpert advice I can offer: wind and strength improve quickly; connective tissue improves very slowly. Older people need to be patient while the joints and supporting tissues grow strong enough for the added load.
I think a good trainer is essential. Someone who specializes in older clients.