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A moderate weightlifting program helps marathoners strengthen muscles that aren't used in running. If running is your only form of exercise, certain muscles, such as the hamstrings, will become stronger, but the quadricep muscles will not. This imbalance can set the stage for an injury.
When you are working out with weights, try to use the same or similar amounts of weight for opposing muscle groups, such as the biceps and triceps, or the quads and hamstrings. The weights you are lifting with your hamstrings may feel easy, but that's OK. Your hamstrings get plenty of exercise from running. If the amount of weight you need to challenge your hamstrings is a struggle for your quads, it is better to lift less weight with the hamstrings than to overtax your quads. Once you build more strength in your quads, you will fell comfortable lifting the same amount of weight with both muscle groups.
You don't need to spend hours in the gym for effective weight training. In half an hour, you can fit in 2-3 sets of 10-15 repetitions for a few key muscle groups: