Young cyclist training help

Hi,
I'm 16 and I'm looking to get back into a serious training regime that will allow me to enter into some adult sportives and races of that sort. I have been cycling for years but nothing really serious and I'd like to see if my amateur skills could be put to good use. I usually keep quite fit by running and going on the cycle trainer a few times a week but its nothing that really pushes me. After a one or two month break i can usually come back and do twenty miles riding easy with some hills involved, i have to eat along the way because I don't have much body fat, but i think that comes with being young. I'm 5'4 and around 50 kilos and just want to get back into some proper cycling again. Any help would be greatly appreciated as other sites haven't said anything about training help for you cyclists which is why i came here, thanks
Rhys