Eat healthily. That is to say eat fruit and veg and cut down on chips. Cut down on beer and stop smoking.
Start running. Three times a week with a day off between. Concentrate on distance, not speed, until you can do about 5 miles, then start to build up speed. If you are really unfit start by walking a few miles each day.
On the in between days, do press ups and sit ups. Start with as many sets of 10 as you can and gradually increase set sizes until you can do sets of 50.
I've just started training with a mountain bike, it's a bit like fartlak training, short sprints and long jogs, once you hit an uphill you are working like mad and then once you hit a straight level part you get your "rest" for a want of words. It has been 2 weeks now and I'm seeing a marked improvement already in my stamina and overall fitness.
Buy a Garmin Forerunner. They really help. I bought the basic model (101) and it does what I need it to. Gives an accurate GPS (to 15mtrs) and gives you your time and pace etc. There is a training partner function which I use quite regularly as well. Worth getting if you have a few quid to spare IMHO.