As others noted, the Masters race will be much, much faster and more difficult than you expect.
That said, do not expect Cat 5 to be a cake walk either. There may be some very strong guys showing up for their first races and blowing the field apart. But don't let that discourage you. The truth is, Cat 5 is where they have everyone start, and some people are legit Cat 1s or 2s doing their first races. As you race more and gain more experience with racing, you'll find out where you fit.
Be sure to read through some of the beginner racer threads in the 33 Racing forum and check back to let us know how you do.