Day 1: Timed walk with a watch. Walk until you have to stop for any reason (fatigue, short of breath, pain, boredom). Record the time.
Day 2: Rest
Day 3: Walk half the time of day 1
Day 4: Walk half the time of day 1
Day 5: Rest
Day 6: Walk same time as day 3 plus 2-5 minutes
Day 7 and beyond: add 2-5 minutes every third day of exercise taking no more than two days off per week.
When you are walking 30-45 minutes per day, five or more days per week, you now have a regular exercise program.
It's okay to substitute biking (outdoors or stationary), elliptical trainer or swimming so long as you time your sessions, start slowly and steadily increase your times.