It's true that some serious cyclists use a stationary bike for training but that is only when the conditions prevent them from training outside. This is because training outside is simply better. It is also impossible to quantify the benefits of experiencing new places, sensations, conditions, meeting people, improving confidence etc etc that you do not get from staying in your living room. If you want to get fit, at some stage you really should leave the house.
edit: Oh and didn't Jade Goody do the marathon...?