It’s pretty simple.
you do 2-3 workouts a week, then the majority of the aerobic volume is from elliptical/aqua jogging.
my senior year of high school I had 9 months of less than 10 mpw but I was in the pool aquajogging until I could go for anywhere from 90 minutes to 2 hours a day split into 2 sessions.
as my injury went away I started running a bit more, but no more than 30mpw and ended up running faster than I have ever in my life 1:55 800 and 4:16 mile. sure the shorter distances make this a bit easier…
but it is important to note fitness does not CAUSE fastness, though there is a strong correlation.