Mileage is important, but staying healthy is more important. If you do that ancillary work for a couple of months, you'll have the foundation built to run more miles. Said another way, before you build your aerobic engine with mileage, you need to make sure your chassis is strong enough to handle your engine.
Breaking the Mileage "Rules"
There are many numerical "rules" you'll find online about mileage. The one that has been the foundation for many serious runners is that your long run should be 20 percent of your weekly mileage. For almost every runner I coach, we go above the 20 percent number—closer to 25 percent.
Let's say you're training for a half marathon and your going to workout on Tuesday, do a medium-distance run on Wednesday and a long run on Saturday. Your weekly mileage is 35 miles a week and you run just 5 days a week. If we take that 35 miles a week and divide it by the 5 days you're running each week, that is an average of 7 miles per day. And that's also 20 percent of your weekly volume, yet you must run longer than 7 miles to be able to call it a long run. If you run 10 miles for your long run, that leaves 25 miles for the other four runs during the week. Let's say you get a total of 7 miles on your workout day and 8 on your medium-distance run. That leaves you with two 5-mile runs for your easy days. That's sound training, yet your long run is roughly 28 percent of your weekly mileage, not the 20 percent number you'll find elsewhere.
For marathoners, the numbers go up even more. You need to have 16 weeks to train fully for a marathon, and you need to have at least two 20-mile runs and one run where you spend the same amount of time on your feet as you'll run in the race. None of the marathoners I work with follow the 20 percent rule, as they would have to be running 100 miles per week to have their long runs—20 miles—be 20 percent of their weekly mileage.
Many runners are running 50 miles a week, so that long run is actually 40 percent of their weekly volume. I've had many runners go from 3:40 to 3:20 marathons in 16 weeks doing 40 to 50 miles a week, but making sure to hit at least two 20-mile long runs, that "long-long run," and many 18 milers in those 16 weeks as well. It's worth noting that all of these people did the lunge matrix before each run, and did some challenging GSM after each workout and long run, which means they strengthened their chassis so they could handle the mileage on the long run.
The bottom line is that as you become more serious as a runner, you should probably run more miles. But make sure you have enough rest built into your weekly schedule to handle the mileage, and make sure you're doing enough of the non-running ancillary work to support the increase in mileage.race.