Is cross-training important when training for a marathon?

Yes, it is important to cross-train.  First, use non-weight bearing cardio equipment such as a bike or elliptical to help reduce the impact on the legs.  You can also use circuit training to work both a cardiovascular and strength training session into a short time frame.  When designing a running program, the strength training is also important.  Circuit training is a good way to get the strength you will need while continuing to build on your cardiovascular fitness.  Because of the already high demands on the legs while running, the circuit programs should have limited lower body exercises.  The upper-body exercises should focus on the larger muscle groups with exercises that will keep you in good form.  The circuit should be short with limited rest and high repetitions.  Using medicine balls, exercise bands, and dumbbells allows for creativity in the exercises while enabling you to move from one exercise to the next quickly.  By moving from one exercise to the next with low weights, you can keep your heart rate up. This allows you to reduce some of your running time but still get the cardiovascular benefits.  In a marathon training program some type of cross-training is recommended several times a week.