Hi
Alice, I see you wrote this over 2 years ago now - how did you get on?
I am 53 and only took up running less than 18 months ago, but in that time I've done the Loch Ness marathon twice, a couple of half marathons and quite a few 10 mile/10k races, all on the road.
I too have been diagnosed with the early signs of arthritis in both hips, with the firm advice from my Doctor to stop running marathons immediately or "you'll be needing replacement hips within 5 years".
I've told him I've got to do one more, as I have a place for London 2010 and I couldn't miss the opportunity.
His other advice was to change my trainers every 2 months and to try and run off-road wherever possible. As for the last bit I find that it it is very easy to "jar" yourself by running over unever ground so it's certainly not the whole answer.