I do weights as well but try to limit the number of times I work each group to maybe 2x a week at most as I'm susceptible to injury if I overwork a particular target group. I like to do the spin/abs classes at the gym as they give me a good anaerobic/aerobic workout. Our instructor for example keeps us on a good sprint regimen, standing and sitting, and towards the end she makes us dismount and do some serious squat exercises and fast running on the spot.

When I get on the trail, I'm suddenly aware how those classes translate into better stamina and that I've found I can climb much better as well - so she is doing great things for my 'pistons' but that's maybe the way it works for me and not for everyone.

I guess the best I look I weights is that as my muscle mass increases then so I had to switch from weight loss (my original objective) to inch-loss (which is more practicle). Arnie might have amazing anaerobic capacity, but I reckon on an mtb I'd probably outdo him. For once, I could tell him "I want your clothes, and the keys to your mountain bike lock!"