Murry
Adams suggestion of TBEC is a very good one. I was a member for several years and have actually just rejoined (I'm racing on Sunday as it happens) but whether this will help really depends on where abouts you are.
TBEC events are generally located in the east and south east of the country.
Generally though I would agree with the other comments, there's no substitute for entering an event and learning as you ride. I know this can seem intimidating but if you approach it with the right attitude it doesn't need to be.
Unless you are an exceptionally talented individual you are not likely to be that competetive in your first event anyway so just go along and see how you do. Enter in the Sportsman class if there is one, don't take it too seriously, stop if you want rest, watch other riders etc (but don't get in their way) and generally treat it as learning experience.
As for bike skills I don't actually think that this is likely to be your problem. I would say that most average trail riders could quite happily tackle an enduro from the riding point of view (especially a Hare & Hounds type course). It is the physical stamina required to ride hard for long periods that gets most people.
If you want to improve your chances of a decent result you would probably be better off training down the gym rather than on the bike but lets be honest, thats very boring isn't it.
Which is of course why I can't be bothered and also why I am likely to collapse in a big heap on Sunday and put in a miserable performace
Good luck and give it a try, it is hard in the physical sense but very rewarding and completely different from trail riding.
To be honest I think that if more people did this there would be less people riding like idiots on green lanes. Why upset people by playing racer on your local byways when for a few quid you can join a club and enter a real race?
Cheers
Richard