The 5 percent rule is a simple way to minimize your risk getting a running-induced injury: Keep your mileage low if your current body weight is more than 5 percent over your ideal weight.
With each pound of body weight you are loading 5 to 8 pounds of force across your back, hips, knees, and ankles. For example, say your ideal racing weight is 150 pounds. If you are hitting the scales at 157 today, you’re 5 percent over goal racing weight. Now take those 7 pounds and multiply them by 6 pounds of force: You are loading 42 pounds of force across your joints. In a word, ouch!
Get yourself strong and in shape before you start logging serious running miles. I suggest biking, swimming, and strength training with only two or three runs per week until you are close to your target weight.
For strength building, I’m a huge fan of the plyometric jump squat: Start with your legs wider than shoulder-width, squat down lower than knee level and jump straight up into the air, making sure that both feet leave the ground. Do seven to eight sets of 15 reps three times per week to build strength, promote cardiovascular fitness and help with the spring weight loss. If you aren’t panting like a dog, you aren’t working hard enough.
If you can build your cardiovascular fitness and strength without loading too much force across your joints, you’ll be happier and healthier come July and August.