Bump up your mileage and eat more (I know this is challenging on a mental state because as runners we tend to have a food obsession, at least from a personal perspective; if you are aware of your caloric intake, you feel as you are doing something bad when you once in a while is eating more than you are training.)
However, I can promise you that you will experience a real boost.
Anecdotally, I know a few runners who have bumbed up their mileage and at the same time increased food intake. Some of them went up some few hundred grams to a kg, others didn't. Common for all of them was that they experienced a huge progression.
If you gain weight, you can always lower your mileage for a week or two and eat a bit less and be back to your match weight. That is okay, because then you have reached your goal. I can almost 100% percent promise this will make you a much faster runner.
Sondre Nordstad Moen, the norwegian record holder is known to gain a few pounds when increasing milage just as a injury prevention and so do many top athletes. One reason is that you are recovering way better, have better sleep because of energy availability.
As said, if you manage to increase mileage and have good quality and at the same time manage to sustain (if you have a fast metabolism) or even gain half a pound, this will make a real difference. Do this for perhaps 2 months, then take one or two week with less training and cutting back on the calories and loos the gained weight, and you are now in a better shape than you had even hoped for!