A horse riding question for a beginner like me?

Try doing sit ups. If your back hurts, you aren't using your stomach enough. That will help your back. Make sure your stirrups are even since that could hurt your back too. Stand on the edge of a stair with your heels hanging off. Dip down as deep in your heels as you can get. Then stand back up on tiptoes. It's a great workout and stretch for your calves. Also you can try running or other general exercises to improve your core muscles and stamina.