After much experimentation I've come to the conclusion that more frequent walks are the most beneficial for me.
I usually run 4-5 minutes (I don't time it, just whenever I feel like it), then walk 30-45 seconds. This works quite well for me.
In a marathon I make sure to start the walks before the first mile. Yes, I'm slow, over 5 hours, but usually about mile 22 or 23, I quit the walks and run continuously to the finish. Typically, the last 3 miles are my fastest, and I've never been passed in the second half of my 19 marathons and ultras.
Hopefully, it won't be too late for you to incorporate walking, I've found it very helpful, and I recover much faster than when I ran the entire way.
Good luck to you.