You likely won’t feel all the muscles you’ve used during the ride until the next day. It’s ok if you feel sore. Stretch your quads and hamstrings and chill. Today's your day to relax and let your muscles recover. It's a good idea to get in the habit of taking 1 or 2 rest days a week if you start biking a lot.
Pull your knees into your chest to stretch your back. Personally, we love watching GCN videos to learn cyclist specific stretches.