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Try to do some type of strength training twice weekly, with at least one day off between weight training sessions. You'll not only get stronger, you will be able to run longer and faster. As long as you keep the weights at a moderately challenging amount, you are not in danger of bulking up to body-builder proportions. If you already lift weights, stick with your normal routine and it will help your marathon training, but don't schedule a weight training session on the day before or the day after a long training run. If you lift on the day before a long run, you may be unnecessarily tired, and if you lift the day after a long run, you will be tired and at greater risk for pulling a muscle while you are lifting.
If you are new to weight training, ask someone at your local gym to show you the correct way to use the machines and free weights that are available. You don't need to do more than one or two exercises for each of the main muscle groups, so try a few different machines and decide which you like best for each muscle group.