Here’s the second key to increasing weekly mileage in a safe but meaningful way: Take occasional down weeks before lighting out for the next milestone.
In the example from above, you’ve gone from 30 to 40 miles a week all at once, then held that new level for two or three weeks. Now, go back down to the weekly mileage you used to run; in this case, follow a few 40-mile weeks with a 30-mile week. This “down week” will give your body a chance to consolidate the gains you made in increasing your mileage. During the down week, keep most runs easy, even if you start to feel raring to go by the end of the week. Save that energy for when you return to your new higher level of mileage.
After the down week, you could either hold at your new level for a few more weeks, or take another leap in mileage, like up to 45 a week for a couple of weeks before taking a down week of 35–40 miles. Keep working this stress-and-recovery approach to your weeks as a whole until you’re at the level of mileage your body, mind, and outside life agree is best.