You'll LOVE it! Just relax, try to focus on what the instructor is saying and enjoy it! As Renny says, you may experience some soreness. There are some exercises you can do to help stretch out your calves before you start. Any exercise will help, actually. The fitter you are, the better you'll be able to engage the muscles you need.
Also, take it slow. No rush. Best to learn to do things right than be in a hurry to gallop down the road.
Make sure you show up with all the appropriate riding gear. Are you riding English or Western? Did your instructor tell you what you needed? A lot of beginners will take shortcuts and try to ride without the right kind of pants/boots, etc. and end up being very uncomfortable. I'm not suggest you invest a fortune in riding gear, but maybe check out some used tack sales once you know what you need. Again, appropriate pants and boots are essential for English or Western. A helmet is highly recommended and required by many instructors these days. A good pair of riding gloves can help too - or just plain gloves that have a good grip and are made of fairly thin, flexible material.
Please come back and tell us how you're doing! There are a few beginner and semi-beginner adults here, including myself! Though I'm a lot older than you :) If I could do it at 45 after 20+ years without horses, you certainly can at 31!