If most weeks you’re running less than four times, then simply add another day of running, with the distance toward the shorter end of what you usually run. So in our example above, our hero could add a fourth day of running to his week and immediately be at 16 miles per week. As I noted earlier, keep the pace easy on both the new run and existing ones.
If you run most days of the week and aren’t interested in additional runs, then add a mile or two, or 10 to 20 minutes, to each run in a week.