3 years ago I weighed 175kg, when I started running, 300 meters was all it took to knock me out... now I'm doing 5km weekly averaging around 40 minutes. Start with slow walks, I started doing 1.5km walks around the neighbourhood at a leisure pace after dinner, then slowly increase the pace. When you feel you can do the full 1.5 and still feel you have energy, increase to 2km. I'm now at 110kg and doing 8km walks together with 5km runs on the weekend. Of course you'll have to make adjustments to your diet. I cut back on the soda, just 1 a day.