Draw 12 checkboxes on a piece of paper. Keep it by your toothbrush.
Pick one comically easy physical activity (walk a very short distance, lift a weight once, do one minute of yoga, etc).
Do it once a day for 12 days, checking a box at the end of each day. When you check all the boxes, draw 12 more.
Increase the amount of exercise slightly each time you draw more boxes.