I increased a couple of my daily single runs from 45-60 minutes to 75 minutes.
As a coach, I run with the kids (but often have to pull my runs short for the slow, low mileage kids--at 45 minutes--to supervise their post-run stretching, and then the rest of the kids would trickle in from their runs to stretch too).
But, after the kids all went home, I started going back out & doing another 15-30 minutes. All very slow miles (because I'd already cooled-off from watching kids stretch & get picked-up after practice), but I was okay with that. Slow slogging.
I noticed after a few weeks of doing that, I felt so strong!
M: 60-75 minutes total
T: 60-75 minutes total
W: 60-75 minutes total
TH: 60-75 minutes total
F: 60-75 minutes total
S: 80 minutes steady (long run)
S: 35-45 minutes easy
But, I wasn't doing workouts. I simply added some extra slow miles. That's when I noticed my biggest improvement.
(I remember a couple years prior, I experimented on myself doing low mileage two-a-days. 30-35AM & 30-35PM. I found working a bigger base, like above, in singles worked much better for me. Those low mile two-a-days didn't give me any endurance. I was just tired. The PM run was always really slow. This experiment on myself simply meant lots of chafing, lots of bathroom visits, lots of laundry. But, no endurance.)