Whew...Well let me start by saying I am not 19 any more
I went out with the intention of doing 4 X 2K today, although I was still having tenderness/soreness on the inside of my right tibia.
I warmed up, stretched out headed out for the first 2 K loop. I am not doing these on a track, I am doing them on a 2K loop on reasonably flat road / side walks.
I went out way too fast for the 1st K of the first rep doing it in 3:36 then slowing a lot for the 2nd K. I took my rest then started the 2nd rep but mentally wasn't in it so stopped after 1 K.
Did a 3rd 2K loop quite a bit slower then the first then decided to call it quits for today partially because of shin pressure I was feeling and also it was a good learning experience but didn't want to hurt myself.
Rep 1 - 7:52
Rep 2 (1K) - 4:17
Rep 3 - 8:10
This was an excellent learning experience since it's probably the first real planned interval training I've attempted in 10 years.
I will know for next time to take it far easier in the first one and it's not a race. I believe I could have done all 4 maybe not in 7:50 but in the 8 :00- 8:15 range which I feel is respectable given my past to races have been in the 4:16 / K race pace.
I am little concerned though about my shin. Tomorrow is a day off... and pending how I feel on Sunday I will attempt the 1.5H - 20% (as you suggested) easy long run, some where in the 1:10 - 1:15 range.