Be consistent.

“Be consistent, make a commitment to walk/run at least 4 days a week. That’s every other day and try and cover about 3 miles. Start by walking the 3 miles the first week. Second week, after you start run a block or whatever you feel you can do until you need to walk. Walk till you feel you can run
again. The distance of the run is not important. By the fourth week I believe you will be running the whole distance.”

Bill Torrey
Rock City Running