Hi! Welcome to TE, and good for you for getting started on biking!
First of all, I don't think 13 mph is that slow. Please don't worry about your speed at this point, just enjoy being on the bike. For the next couple of weeks, you probably want to keep your pace moderate and gradually increase your mileage. This will help your body get used to biking again with a minimal amount of pain.
Keep it fun! Is there someone in the same situation (trying to get back into biking) that you could go riding with sometimes? That can really add to the enjoyment of riding.
- Melissa