And your kids will always remember that it was Mum who ran about and had fun with them! Not only that, but while I am no great athlete, my son has always been used to seeing me exercise and accepts this as normal. We're setting them a good example too!
I think it's an age thing unfortunately. The hip pains you describe are the first signs of wear and tear in the hip joints, but the best thing for hips is to keep them moving through the whole range of movement. Stretching too is so important when you get older (I'm 46) and can help to keep the stiffness at bay and keep you flexible. However, I think we do have to accept that bodies do deteriorate and some aches are inevitable.
The couch potatoes don't get stiffness/aches because they do not challenge their bodies in any way, but they cannot do the things you do or have the energy you have.
Like you, I enjoy being able to go out and run, run up stairs, play with my son etc without getting exhausted/ breathless like the couch potatoes and I think the occasional stiffness is a small price to pay for this.
Glucosamine and cod-liver oil is a useful supplement for all joints and I have found it helpful. Exercise does help, only people who don't exercise ever say otherwise, you were unlucky with the cancer and good on you for working hard to regain your health and fitness.