Time to put on the dull Sports Scientist hat.... As a general rule cycling fitness transfers well to running but not so much the other way round (why triathletes skew their training to the bike). The reason for this is due to muscle fibre recruitment. When you cycle every peddle stroke is like pushing a weight (obviously more so when going uphill).. that "weight" includes you and the bike. Add in additional resistance from friction and lost energy through gear transmission etc and you'll be recruiting most of your fibres. Running simply doesn't need so much recruitment so the legs don't get as strong. That's why a runner can get on a bike and feel fine in the heart and lungs but their legs will be toast.....