My reaction is "Do what YOU want to do" If you would like to lead the group or even just be a part of it, DO IT! It sounds like mountain biking had been a big part of your life and it might be nice to get back into it. I'm tempted to say: Don't let the young farts intimidate the old fart -- but I know how it is... But remember, the last time they couldn't even find their way out of the woods!

Actually, its quite a complement that they would even ask you...

Or, maybe you would like to do it simply to help out and do something worthwhile... That's OK too...

But, if you are doing it because you feel pressured into it, that's a bad reason and you should say NO! You do not owe them anything. You have no obligation to them. It reminds me of a line from a David Heinlein novel:
"If tempted by something that feels "altruistic", examine your motives and root out that self-deception. Then, if you still want to do it, wallow in it!"

But too: What about your back? The last I heard you were recovering from some back issues?