Taking up the bike is a great way to stop smoking, as I found out over 2 years ago.
You will get into all the 'bling' that goes with cycling- so a good motivator is to think of all the shiny bits and pieces you can get for your bike with your old fag money !
You should find that after perhaps wheezing up hills to start with you eventually develop a steady breathing rythm.
As has already been said,build up your rides progressively, and the weight should drop off you.
I was no more than 12st 7lbs when I returned to the bike, but am now a steady 11st 2lbs.
You should find the hills get easier as you lose weight.

Enjoy