Each person's time table may be different, but the steps (pun intended) in the process are similar, regardless of the fitness level when you start.
Walking is the base. Build distance slowly. Add some jogging - get doctor's okay. Again, build distance slowly. Have fun. If it's not fun, then find some other activity that is fun.
Pacing yourself is absolutely essential for distance running, or any long activity. Being out of breath may mean that you going too fast, not that you have reached a limit. Alternating walk/run has helped me slow down. After walking for a minute, my heart rate drops, my breathing slows, and it's easier to resume at a slower pace. Alternating is easier on the joints and muscles as well. Remember to get plenty of rest and drink lots of water
I was a couch potato 3 years ago. Couldn't run 100 yards without gasping for breath and taking a nap. I didn't start running until I had been walking for several months. In that time I built up to a 12 mile walk and came to two conclusions.
1. I can cover the same ground faster and burn calories faster if I were to run.
2. Get a new pair of shoes - good ones at the running store.