Hi Mags,
I live in Tamworth NSW and am 39 yrs. I have had LARS ligaments in knees, disc L5 issues but have found CrossFit is excellent. Find a local gym and do the beginners program. All of the workouts are scalable to ability/strenght.
Many of the movements are very benifitial to Trials. Light squats, Box Jumps, Ring Rows and Dips all strenghten muscles that will make your time on the bike easier and more enjoyable. I have been training with Crossfit 2340 for about 18 months and have found my bike fitness to be so much better.
During the winter months i work until dark so actual riding is limited to the weekends unless i can rack a day off somehow. I like to practise balance and front and rear wheel hops without the engine running.
Training for fitness will also reduce your body weight making the bikes job easier, ride for longer and also help you enjoy trials into your later years. And ride. Join a club and ride trials.
Have fun,
Jeff.