I love reading threads like this, keeps me motivated! Good to hear it's getting easier for you (and Sittingduck), it does start to get a bit more enjoyable doesn't it.
I unfortunately can't commute but after pottering about for ages getting used to being on a bike I started to try and go out as regularly as I could and after a few months I really did start to feel as if I was getting somewhere. I haven't lost any weight at all, but was certainly feeling fitter, and though I still haven't been any great distance the 13 mile circular route I had done four times was starting to feel pretty good.
I've had a bit of a setback after being without my bike for a few weeks while it was getting repaired. Just been back out in the last couple of days and not only am I knackered after a few miles (and any kind of hill), my backside is sore too
So I'd recommend sticking at it and don't take a prolonged break!