Running, stretching and core work.
Start with brisk walks building up to jogging and then running, remember to stretch afterwards to help improve your flexibility and to help to keep your muscles from feeling sore and tight.
Pilates and yoga are both excellent for strengthening your core, improving your overall strength and flexibility, and an overall evenness/straightness on both sides of your body. They also help you to become more aware of and in tune with your body. Pilates and yoga can be extremely beneficial to your riding.