I'm fortunate where we live there are superb cycle paths; cycling to work and back is one of those things that , on the way home, you don't have to have any motivation, you have no choice!
I live off the buzz I get all day at work from a good bike ride, rain or shine, MP3 on full blast. It's also a great time-manager as you don't really need an awful lot of extra time, think of all the traffic you avoid
i would start off with simply walking fast around the block, every day -
if you try too hard straight away it will put you off as you will struggle, so start off gently
as you start to feel better, start adding other things like going round more times or run around the block, perhaps carry weights, then start some kind of planned routine like aerobics or something - you will be more likely to keep it up if you don't push too hard
Try to find an activity that you like!! If you join a gym but hate every minute of it, you will eventually give up. If you can't think of anything that you like, then try alternating, so that your motivation is kept interested! ie, cycle one day, walk the next, run and walk the next, swim etc,etc.
I got a cheap mp3 player, put my fav. music on it and started walking places. When you are listening to music the time passes quicker and if it is quite fast it speeds up the pace you walk at. good luck