Just one other thing I'd throw into the ring while it's fresh in my mind.
A friend of mine isn't overweight but was a regular smoker for many years. Recently he quit smoking and started cycling to work. His commute is only a couple of miles but does take in a fairly unforgiving hill. He and I normally go out for a ride every couple of weeks or so.
Not so very long ago the effects of his smoking were plainly visible. The slightest incline would leave him struggling, he could only manage 8-10mph even on the flats and after a few miles he'd be wheezing. Having stuck at it (and quit smoking) the results are equally visible. These days he'll hold 15mph or so on the flats, is significantly stronger on hills, and yesterday he and I covered slightly over 15 miles before he started to flag, and then the signs of flagging were that he slowed down a little rather than got off the bike to gulp air.
So stick with it... at first you'll almost certainly be like my friend was (and like I was earlier, and like most of us in here have probably been at some point), but over time you'll get stronger. None of us got to weigh ~300 overnight, and it won't come off overnight either.