If you have not been riding on a regular basis, your bike may not be as strong or as flexible as it will be after a year of riding every day. As a temporary solution to backpain, get the seat and handlebars closer to each other so you can sit more upright. Your back should be at about at 45 % angle...if your nose is down by the stem, you are too stretched out.
Some handlebars are shaped like an up-side-down U. That brings the grips back closer to your waist, and lets you sit more upright. That also puts more of your weight on your "sit bones" and less on your hands and wrists. Work with your LBS to get a fit that works for you. As time goes on, you may be able to handle being lower, and more stretched out.
Try to ride at least every other day. That will help you get your muscles in shape. Riding just on Saturday is a recipe for being sore.