I just gave my bike to my little sister (part of accepting that it's not like I'm going to be able to use it again). But it's surprising how easy it is to keep your weight and shape more or less under control just by not eating vast amounts of crap, and by doing teeny-weeny low-impact exercises little and often. My physiotherapist gave me a bit of this stuff called Thera-Band, see if you can nick a bit - basically it's stretchy stuff you play with to do resistance exercises, tie a loop of it to the desk or bed or whatever and that's fun. Then there's all sorts of posture stuff and stretches that help. OK so it doesn't feel as active as when I used to cycle or rollerskate or dance, but I've been basically sat on my arse doing a grand total of buggerall for over a year now and I'm still wearing the same size clothes.