Paul,
I am a coach and cross training is important. We cross train twice every 4th week to allow my athletes to adapt to the next training level. Cycling and the cross trainer are the main ones, a little circuit session specifically targeting running/non running muscles. Cross training will be done progressivly to avoid soft tissue injuries. You can set targets/aims for different cross training sessions. An example, if my athletes have done a 3 week cycle for power, then the cross training will target power exercises.
A good thing about cross training it helps variety to your training with specific aims. Do not do too much cross training if you want to be a runner.
Have a nice run tomorrow.
Allan