Everyone is different. I stressed my joints a lot in my teens and 20's (lifting and carrying weights that were extremely heavy for my size and a lot of mountaineering with steep downhills) and tend towards arthritis, so in my case, quitting running was a no-brainer.

The years of running, regardless of age, may be a bigger factor. I started running and backpacking when I was 16. Back then we had horrible running shoes- no cusioning at all!

All I am saying is that you can stay in shape for backpacking with less stressful exercise, if needed.