Well first off running is not the same as cycling. Your running will give your legs a good work out and it is aerobic, so there is a certain amount of carry over but it will not be 100%. The motions in running and cycling are different enough. Shoot, I recall one year I was doing a fair bit of wind training on a stationary bike. After my first road ride of the year, my legs were sore in a few places. I was hitting parts of my muscles on the road in a slightly different way than on the wind trainer. Doing a bit of 20 mph for an hour is not bad especially given the fact that you have been off the bike for 4 months.