i did have one which goes on the bootlid it had clips to attach to underneath the car and straps which came through the top of the boot and also straps to go onto the side of the bootlid. Worked fine plenty of times now the ex girlfriend has it and she doesnt even drive!

I'd suggest you turn up at the Ashton Court Golf Kiosk (see map) on a sunday morning at 10.30, maybe post up here to double check first. Local rides out of Ash Ct are usually around 10 miles of muddy singletrack ,taking 2 -3 hours at a mellow pace. That way, you can guage your fitness against others and see how you're doing. Also, you're never far from a short cut home if you're knackered.
Hope this helps, Giler.