I used to run for fitness but cartilage ops on both knees have made running a thing of the past, on doctor's advice.
I'm about 9 months into a return to cycling after a break of about 10 years and loving getting out and about - it's now my main form of exercise. I think there are quite a few ways help stave off Osteoporosis, starting with good old press-ups/sit-ups, some home weight-lifting and social walking. I'm hoping that will be enough for me!
Incidentally my wife is a Coeliac - allergic to gluten - and this has the effect of blocking the gut's ability to absorb calcium, so she is on a high dose calcium tablet daily having received a marginal bone density scan. The body also needs Vitamin D to handle the calcium effectively, so make sure you get a bit of sunshine...