Lots of sound advice above.
My two best nuggets of wisdom for you are fairly basic if you want to get into road racing:
1) learn to be comfortable/confident in a fast-moving, tightly packed bunch. A lot of fit guys and time-triallers assume they can automatically road-race and cause a lot of crashes cos they can't handle a bike in those circumstances. As such 4th Cat races are generally clumsy affairs (so don't take a nice bike!)
2) As someone else mentioned above, you gotta have a 'jump'. If you haven't got one, you gotta get one. There's a lot of pace fluctuation in road racing (certainly in the lower levels). Whenever the pack kicks off, you gotta go with them or it's game over.
Only done a couple of XC races and they were more like time trials. TBH. Or a two-hour 'Cross race. They soon evolved into individual riders just going as fast as they could.
I suggest *strongly* you come and have a go at a 'Cross race There's one at Ashton Court next Sunday. Email me for details.