In reply to paul walters:
Yes, I've seen a physio who recommended shortening my stride, and trying to land further forward on my feet (all of which has helped) though I never landed on my heels anyway.
I bought new running shoes from a specialist running shop after having my gait analysed.
In addition to the running I use the ATM, stepper and treadmill at the gym twice a week, as well as get out onto the hills as often as I can on weekends.
A friend (with a longer history of fitness than me) did MB last summer and struggled with the altitude (though the group spent no time to acclimatize), but my concern is I won't have the base level of fitness to enjoy (or even complete) the ascent.