Comments about "accumulative aerobic" , "all running matters", "training must be difficult", .....
Young runners cant see the forest thru the trees. They are so focused on today tomorrow this season or next season they don't see the big picture. What happens in a race today was determined not only by recent training but also training you did months and years ago.
Training adaptations occur all the time not just in response to a single bout. Run 5200 miles in year and your body responds accordingly, likewise run a single bout be it intervals or long run and your body responds accordingly.
Doubles allow two things: improved recovery while still maintaining or increasing overall volume (run 5/5 vs 10 when necessary for overall recovery), increased overall volume when a long single would be too hard to recover from (5/10 vs 15).
All running provides some benefit, even if it's just weight management.
I was never near malmo's pedigree but I did the best with the hand I was dealt by looking ahead and having faith in my training.
The OP asked when he should see benefits of base...they are already there and will show themselves even when you aren't looking such as in the ability to now do more volume in season more intervals more volume next off season and the off season after that and so on.
Alan