I don't have time to run more than 30mpw during the winter (light levels), so after my track/xc training at school I hop on a stationary bike for about 45mins. On easy running days, I do a short interval workout on the bike, and on hard running days I do an easy ride on the bike. I started this about 2 weeks ago hoping to get some consistency since my school training is irregular. My coach thinks that long runs or high mileage will kill my speed, so I figured that adding an equivalent time on the bike every day would be like doubling my mileage. Instead of 25-30mpw, it's about 45-50. The problem is that to get my heart rate up on the bike I have to pedal faster or increase resistance, and statipnary bikes do not take resistance into account when determining speed.
TLDR, I like doing the bike and I hope it works.