Hello numerical pieperson . <waves>
Hmmmm. I would suggest that you check out the course, looking for hills, tricky bends and anything that might slow you down. Then ride it a few times and if you can, time yourself each time. 5 miles isn't very far but it will seem like it when you're trying really hard, so the best way to build up your speed is to practice and get in as many miles as you can. Do you know anyone who is faster than you (friends, brother, sister, whatever)? If you do, get them to ride the course ahead of you and work as hard as you can to keep up with them. They will beat you, but you'll get closer to them each time and that's good training for you.
The other side of it is your mental attitude. Don't be put off by competing against older kids, you want to do the best time that you can manage and if you do that, where you come in relation to anyone else doesn't matter. Besides, if you work at it then you might surprise yourself.
Oh and have a look at some time trialling forums. You want to be in a good position on the bike (head down, arms tucked in) so that you don't create too much air resistance.
Let us know how you get on....
