This article is from the Misc Bicycles FAQs, by various authors.
Drink plenty of fluids. Get into warm dry clothes. And celebrate with
an ice cream sundae or some other high-carb high-fat treat!
You may also want to reward yourself with a massage. Some organized
rides will have massage therapists working for a small fee or for free
at the end of the ride. If you live in a large town, there may be a
massage school, where for a small fee, a student will practice on you.
(I found my current massage therapist this way :)
Then start planning your next one, because you will be hooked!