It's funny how we all chime in here, and genuinely how different we all are, even if we're running similar times..
I think I am polar opposite of you my man, I've only ran 40+ mpw maybe less than a handful of times, I am 25-35 mpw year round the past 3 years. With a young child, flirting past 35 mpw starts to kick in the fatigue syndrome while chasing the lil one. Once he gets a lil older, I'd like to get into the 40-50 mpw zone to really see where some of my times can dip to, as long as I stay healthy.
I believe I must be more fast twitch oriented, I would love to actually know this for sure..my 5k pr is 19:12, just clocked a 19:22 last week..one of these days I'll crack sub 19..getting close..
I am just trying to keep the mileage consistent and stay healthy. I find a lot of value in getting out the door almost daily, and working on strides after almost every run, 5-8 x 1-2 min 5k paced intervals interjected into steady paced runs. I like mixing a tempo effort into longer weekend runs. Full body strength work 2-3x a week.
I guess staying healthy is the biggest hurdle, best of luck!