Back to the original question...
The Juniors races are usually FAST. Many of the kids have been training all year (road season). Some will also run XC for their schools, or play other sports. So, being on the slow end of that field is pretty meaningless at this point.
Training...
1. Find a local cross practice. Ask around the local shops, or email some of the roadie teams. Usually mid-week, high-intensity, and on grass.
2. Race twice on race day (assuming only racing one day). Enter the Cat4/C race and the Junior race. Take the next day as recovery (if you can't go slow/easy enough on the bike, go for a long walk or an easy swim).
3. Add in one or two longer, easier rides (see FatBoy's comments about being young and having fun).
#1 and #2 gives you two days of high-intensity. That's plenty for somebody new to training (heck, it's plenty for many experienced cyclists too).
#3 gets you going with some base fitness that will carry over into next year. It takes years to build the level of fitness required to race at the front.
Your week might look something like:
Sunday - race x2
Monday - recovery (super easy ride, or walk, or swim - and I do mean SUPER EASY)
Tues - slow, fun ride
Wed - group cross practice
Thurs - recovery (same as above)
Fri - slow, fun ride
Sat - leg openers (easy, short ride, with a few hard sprints thrown in to prime your body for racing the next day)
On recovery days, if you feel tired, overly sore, don't get on the bike. Walk around the block and get to bed early.
At your age, one of the biggest problems you might have is getting enough sleep during the week. As noted above, lots of intense training without enough sleep is a recipe for catching colds.