Hi dorymai
I agree with Morwenna and the others - walking, outside, fresh air - that's for general health and fitness.
Also at our age it is important to do other exercise to help our joints - keep them supple and mobile. I have just started Pilates for this and it feels so good - very gentle exercise. it also helps strengthen the core of the body and to gradually build muscle.
Finally if you want to lose weight then replacing fat with muscle has the added benefit not only of keeping you fit, but muscle burns more calories at rest than fat so more muscle increases your basic metabolism so you can eat more and stay the same weight. I go to a body conditioning class for this - colloquially known as "tums, bums and thighs". This is also brilliant. An hour flies by - but it's hard work!
I used to do fitness videos but I find it so much better going to a class because at home I have to motivate myself and there is not enough room to leap about in the room properly anyway!
By the way this has also helped my joints which a few years ago had started to become a bit stiff (in my early/mid 50's) and now are no longer ( I am 60). HRT may well have helped as well - but I don't want to find out by stopping!
Hurdity x