If you’re running a few days a week, is that for time reasons? If so—for example, if there are only three days a week where you currently can find the time to run, but have a bit of wiggle room in those windows of opportunity—then run more on your running days. If your schedule is super crammed, you’ll probably more regularly run more by adding 15 minutes to each run than by carving out a devoted-to-running block of time on another day.
If you’re running a few days a week but could find the time on other days, then start by adding another day of running each week, and keep the distance of your normal runs the same. Once you adapt to the increased workload (which probably won’t take more than a few weeks), weeks with four days of running, or five days, or whatever the new amount is, will feel like the new normal.
If you already run pretty much every day, then start adding additional mileage to your medium-effort days of the week. On days when you do hard workouts, you could add a mile or two to each of your warm-up and cool-down. Then you could make your longest run of the week a little longer. Tack on mileage on these days before running more on your easiest days; keep those really easy so that they serve as true recovery days instead of becoming another medium-effort day.