I appreciate this isn't mountain biking, but so many of our clients and myself (jpj) run for fun, but also as kind of cross training. If you haven't seen this you may find it interesting.
I personally tried this after years of knee pain and so far so good! I'm not 100% convinced but the proof is in the pudding and I'm now running after years of having to avoid it. Am I causing stress elsewhere? Is this going to lead to future issues? I don't know to be honest but at least I can now find out!
For years I was told by high end cyclists that running and cycling didn't mix as they use different muscle groups and riders develop un-required bulk. This I have decided is a load of rubbish unless you're already in the top end of racers and becomes less important when looking at downhill and gravity enduro riding. The benefits you can gain from running, in my humble opinion are massive and if you need a quick turnaround for the old heart and lungs running can be a great addition to a training schedule, certainly in cross training season!
Not quite ready to drop my shoes yet though :-)