Ok….
You can build fitness/stamina quickest or most effectively by riding on the road; you get a more sustained effort for the same time.
Get in touch with or carry on riding with the local road club; they may seem like old duffers, but if you can take in what they say, and take in the bits that help you while disregarding all the old wives tales, you will get fitter.
Do two things; aim to get fit by doing rides as long as your races, and aim to get fast by doing interval and threshold training sessions.
Do as many races as you can, initially not to win, but to get your race day prep and race strategy sorted.
Concentrate on improving your skills as well as your fitness.
Enjoy it as much as you can, you may find you have unlimited energy, but you only have a limited amount of enthusiasm; don’t use it up too quickly.