Down here in south florida, to compete in cat 5 you'll need to be very fit. You'll have to be able to handle a race length of approx 50 minutes at an average speed of 25 mph with constant surges, accelerations, and high speed / tight corners. clicking the 30 mph mark many times during the race is normal.
=)
To get a feel for criterium racing without actually being in one, I suggest you try out the training dvd named
" Race Day ".
This is an on-bard camera training dvd with a virtual dashboard that shows you what cadence and effort you should be applying and it is the full length of a typical criterium race. It helped me a bit and it should be of some use to a beginner to get an idea of the demands of racing.