Marathoners are not known for lean bodies so listen to how my "lean and toned" friend Janet cross-trains for the Boston Marathon. You don't just go to the Boston Marathon---you have to qualify for this very prestigious event (happens this Monday).
In short, Janet cross-trains her body and she just happens to like running alot! Just yesterday, part of her workout was to ride on a bike. Janet also regularly strength trains, swims and of course, runs.
Personally, I don't ever see myself running a half marathon or marathon. But, if I ever decide to train for a marathon, I will cross-train like Janet. I would want to be a "leaned-out" marathoner like her!
So, here are some cross-training tips for runners:
1) Lifting weights should be a big part of your training regimen. Two to 3 days a week of full body weight training will strengthen your core and overall body. A strong core is a prerequisite to being a strong runner. A strong body will also help you recover faster from races and injuries. Plyometric and speed training should also be a part of your overall training program. Having more power and speed will give you an advantage over your competition. Speed and plyometric training increases your chances of injury. Proper warm-up is critical. You could include one day each of speed and plyometric training (not consecutive days).
3) Don't over-train your body. Research shows that the lowest number of running injuries occur when you run three days a week. Each added day increases your chance of injury. Also, don't run too many miles in a week. Those who run 20 miles or less a week experience the lowest number of injuries.
4) You need rest between exercise days and walk breaks during long runs.
5) Proper warm-up. Dynamic stretching (using your muscles to warm-up your muscles) has been shown to prepare your body best for strenuous activity. A sample dynamic warm-up could be a 5-10 minute jog, prisoner squats, cone/ladder drills, lunges, etc. Static stretches would be done after your workout.