well, i have done some fell runs, but wouldn't consider myself a serious comprtitive fell runner, i've only so far done it for enjoyment and so seek improvement in that area.
popsider: it was kind of obligatory cross training, the cycling, because i was commuting to school and back. however, i have now moved to training twice daily, and at the moment, i am running once, and then cycling strenuously for the other section. so it's not really cross training, but training to be able to train twice a day. i found it benefited my running more that other trainings like swimming etc. however, it's not an alternative, i don't think that for a minute. the best training for running is RUNNING! :-S