I follow Shoey's fitness line of thinking. His comment in "what do I hate the most" thread he mentioned blown out suff, I too am a surfer, more an aged surfer these days and also hate blown out surf, but spending two hours out in the surf paddling a board through the waves certainly gives you a good workout and great strength building, particuarly in the upper body.
I'm sure not many of our fellow VMX riders are into the surf scene, so I suggest swimming. Swimming provides a great cardiovascular and airobic workout routine, plus it's good for low impact upper body strength training and a great way to loose a few kilos to boot. Initially you may not be able to swin 50 meters without stopping for a break, but you will be surprised how quickly you can get in to the groove with a session or two a week and eventually finding youself swimming 1-2 klm in a session.
The saying "use it or lose it" rings very true as we all age, more strength training equals stronger bones and stronger more fleible joints, exactly what we need to withstand those falls that inevertable will come when rideing bikes.