You're right Frustruck. I perhaps didn't make myself clear. I was suggesting that to gauge your fitness (especially for someone who admits they can't climb or walk up hills), you could do the regular route on foot, where a time of sub 5 hours would show if you are even remotely fit enough. Doing it on foot is of course relevant, because on the cyclo cross event most people walk a lot of the up and even some of the down!
Opportunities to actually practice on a bike are very limited. Apart from the road sections, I don't think there is legal bike access to any (?) of it. However there are some bridleways in the area where you can find similar riding conditions, such as rough rock strewn lanes between dry stone walls or steep millstone git steps.
But if you don't know all this, you are probably not experienced enough to take part and probably wouldn't get through the screening process, for what is a very over subscribed event. An alternative is the Hell of the North Cotswolds. It again is a mixed off road / on road event with plenty of hills. Doing it on a cyclo cross bike puts you in a very select minority amongst all the baggy shorts and full suspension.