Begin inserting segments of 5-10 seconds of running every 1-2 minutes of walking into your workout every other day (stick with just walking on the day between run-walks or take a rest day). If all is well after three or four sessions, increase your running segments by five additional seconds each week. When you are able to comfortably run for 30 seconds and walk for 60 seconds, gradually decrease the walking amount by five seconds each week until you're down to 30 seconds running and 30 seconds walking.
To prepare for a 5K (3.1 miles), increase the distance of one run-walk session a week—usually on a weekend—by 5-10 minutes each week. When you have covered four miles on your long workout, you are ready for a 5K. Keep your run-walk ratio at 10 seconds running to 50 seconds walking in order to avoid aches and pains.
To prepare for a 10K (6.2 miles), increase the distance of your longest workout every other week by 10 minutes. On the alternate weekend, when you do your shorter long-distance workout, cover half of the distance of your current longest run-walk. When you have covered seven miles on your longest session, you are ready for a 10K. It's best for true beginners to keep the run-walk-run strategy to 10 seconds running, 50 seconds walking, 10 seconds running, etc.