search around the forum - I and several others have posted a lot of information about how to get started racing.
I don't know if I've ever answered how long are races, so I'll do that now. Generally beginners road races in my area are around 25 to 36 miles long - and don't worry, even if you are a century rider, this will be challenging enough. There are more challenging races, often if not usually raced in combined fields with higher level races. The longest I've done this year was about 101 miles.
Beginners crit races are usually from 25 to 35 minutes long.
TT's for all categories run from around 10 or 12 miles (typical) to about 24 miles for state championships.
People considering racing almost always want to know if they are fast enough to start racing, but pure speed isn't the determining factor as to whether or not you'll be able to stay with the pack in a race, so its not really possible to say if you can do x mph you'll be a successful beginner racer. Even very strong people can fall off the back if they have a hard time handling surges, hills, or just can't get comfortable in the pack, while weaker riders can take adavantage of tactics and good postitioning to get good finishes.