When you combine serious strength training with serious endurance exercise, your body will probably choose endurance over muscle and strength.
The authors contend that if you mix strength training and endurance exercise, the workouts will actually interfere with each other. In general, you will get the benefits from the endurance exercise. If you were working out 3 days a week in the gym and 3 days a week out running, then your body will choose endurance over muscle growth, strength and power. If you did want the best of both worlds, you would need to combine the workouts and do your running and your strength training in the same workout session.
I generally run and lift throughout the year. However, I always have a focus on one or the other. When I finish my racing season, I tend to cut back on my running and concentrate on my lifting. The general idea is to put on a few pounds and to work on building muscle mass and strength. Then, when my marathon training comes back into focus, I will cut back on my lifting to two days a week and start running five or six days a week. The continued strength training is not meant to build a lot of extra muscle mass, but to maintain the strength that I do have and to provide another outlet for improving upon my endurance without further stress that is exactly the same as running.