If you haven't been exercising and stretching regularly all year long, you need some conditioning. If you're not sure how to stretch, there are numerous books available. Check your local library or bookstore and be sure the author is credible. Always begin work with light activities. Then do heavier work and finish with lighter work.
Pacing is important.
Listen to your body. When you are tired, rest or stop. Don't try to do too much work at one time. As your gardening days add up, you should be able to work for longer periods of time. Maintain good body mechanics by trying to keep your back from bending forward, and using your legs for bending and lifting.