Welcome to Fluther.
If you can walk three miles, then you can “do” a 5K fun run. So start there. Measure off a 1.6 mile distance from your starting point (for the warm-up and cool-down) and walk ‘out’ as briskly as you can, and back the same way. Now you know the distance.
Daily, start jogging the distance, or as much of it as you can, after first stretching to warm up a bit, and walking a bit at the end to cool down. When you’ve jogged all you can of your measured distance, don’t stop, but keep walking until you have your wind back, and then jog again. Note improvements.
Don’t press for too much, too soon, or you’ll develop blisters (your feet are still tender from inactivity) or shin splints. Blisters are an injury that will prevent you from even walking, and shin splints, while not technically an ‘injury’ will make you want to have your legs amputated. If you feel too much tenderness coming on, then just stretch and walk that day, but do the whole distance.
You won’t win the race, or your age division, but you can finish it. Take that as your win.