You may find it slightly more difficult starting mid-season, as the more dedicated riders have been racing now for a few months and will have their race fitness already. The best thing to do (as you've already indicated) is to turn up and have a go, with no illusions on how you think you'll get on. If you're not already a member of a club, then I'd also recommend that you look into that.
I'm a couple of years younger than you, and only started racing this season myself. Now I'm pretty addicted to it! I've written a few reports over the last few months that may give something of an insight:
http://www.cyclechat.net/threads/popping-my-big-fat-4th-cat-cherry.97563/
http://www.cyclechat.net/threads/race-report-shred-classic-2-3-4s-plymouth-hoe-13-05-12.101981/
http://www.cyclechat.net/threads/east-devon-circuit-series-race-1.102561/
Your fitness (or comparative lack of it) is not anything to worry about. You can, in effect, race yourself fit. What takes time is the getting your head around the dynamics of the bunch: Where to ride, who to follow, when to put the big efforts in, how to position yourself assertively - all that kind of stuff. I'm not going to offer any advice on that, because I'm still learning myself! Make notes after you race... I love having numbers to crunch afterwards to see how I did and where I can improve. Also record how you felt, and what you think you could improve on. If anybody at the race is taking photos, then also see if you can look at how you're riding to see if there are any obvious improvements to your riding style. Take this one as an example:
Aside from the fact that I'm still a tanker, and still need to lose another 10 kilos or so, I'm also on my brakes, when there's no need to be - I actually corner reasonably well, but I need to stop the temptation of slowing... Also my arms are not bent enough - I'm making things even more difficult for myself by not getting more aero.
Whilst on the subject of aero... I heartily recommend that you take a turn on the front if you can. Put a break in if you have the legs, even. What you need to appreciate is the difference in ability between the people at the front with the engines to actually
drive the race, versus the people who sit in and take advantage.
Above all, turn up, enjoy it and don't be disheartened if you get dropped. Use each race as an opportunity to improve, not to mention an opportunity to have fun!