It is frightening how little you learn about running, despite doing it for years, when your talent is that you could run faster than everyone in MS on little training.
Base does not disappear completely, it might be coordination but it doesn't feel that way. I'm not sure this is the same but it felt like what Magness describes here.
http://www.runnersworld.com/running-tips/aerobic-ability-stays-with-you-as-you-age
In MS, I was slower than everyone on minimal training. After 4 months of 3 miles per day, I ran my first HS XC 5k in just over 22 minutes. I then averaged almost 10 miles per day for the next 6 years. Then, I didn't run for 10 years. One day, I got talked into running a XC 5k without any training. I again ran just over 22 minutes.
With that kind of success, the next weekend after maybe 3 more 3 mile runs, I went to a local running group's long run. As the run got started, I picked out the heaviest guy who was not a real jogger. He was doing 10 miles so I tagged along. The first 5 miles were mostly uphill, we average 7:30 pace. The second 5 were mostly downhill and we ran just over 7:00s. After that it took me more than a year to get my 5k under 18 but the aerobic part was no problem.
In college, I slowly worked my 5k up to being as good as my mile so you could say I had more speed than endurance, but not really much of either. I got down close to 15 flat so maybe you could say my talent is trainability, but I promise that for the 10 years I took off, I did NO exercise.
""Base" disappears completely even in elite athletes if you stop running. Ask any "former D1" athletes like myself.
But then once we start running again, we pick it up much quicker than our peers and can achieve levels higher than our peers. This is not because of some long-forgotten base from 10 years ago. It is simply a matter of better genes and natural ability.
Just like in middle school where we would be the fastest in the school on minimal training, the same applies when we run with the local club (unless there are some talented or very well trained runners in that group).
I stopped running for about 10 years after college, I started running again a few months ago for fun (after 10 years of drinking, smoking, and literally nor running at all), do around 20-40mpw depending on the week, and I'm already doing most of my runs at 6:30/mile pace. For the first month or so I was struggling to complete a 5 mile run at 8:00/mile pace, but now when I go for a 10 mile run I can kick the last mile or so in about 6 minutes. Mind you I used to do 10 mile "easy runs" at 5:45 pace so I've fallen quite a ways, but the talent is still there."