To practice a TT, unless you know the course, is a bit tricky. If you do know the course, then start at the paint line on the kerb, ride as if your life depended on it, not a moments slacking, avoid the traffic , stop and log your time.

You may well be disappointed but the important thing is that if you turn up on a day when dozens of other peopke are competing you'll find you can knock at least 1 minute off a 10, or maybe 2 minutes on a fast course. To race a real 10 TT you will need to join a club or have helpful friends who are in one.

I know nothing about completing even a short triathlon but the best way to get faster at cycling is to do lots of it.