It depends on your fitness level. Specificity is indeed king with it comes to the transfer of training effect to your event. Since all mountain sports we deal with are weight bearing you’ll for sure do well to incorporate mostly foot borne activities like hiking, skiing, running. If a person has very low fitness then any aerobic activity will be more helpful than none, but as one becomes fitter specificity plays a bigger and bigger part. In that instance non-specific workouts like rowing, cycling and swimming will be very effective recovery workouts but will contribute little to making you faster in the mountains. As for running vs steep treadmill hiking: We find running is one of the best ways to build the kinds of specific leg strength for all mountain activities, the hillier the better. You can certainly simulate this on a treadmill or incline trainer.
The transition period’s purpose is to prepare you for the training that is coming. If you’ve been mainly rowing in the transition period but plan to begin running more in the base period you’d be wise to start the transition to running sooner. We find it typically takes about 100 miles of running before your legs are conditioned to handle running training and minimize the risk of injury.
I hope this helps,