I'd explain how a sub 90 has to be easier than a sub 40 like below using my own pbs, and McMillan.

39:59 is 6.26 pace, 1:29:59 is 6.52 pace so 26secs a mile difference.

My current pbs are 75:41 half, and thus 5.46 pace, and 34:30 10k which is 5.33 (13sec difference)

Even taking McMillan's optimistic prediction from that half of 33:57, that'd still be 5.28 pace which is still only 18secs a mile difference.

26secs is a big gap between 10k and half, and is backed up in Phil's point about the 2% WAVA difference.

Therefore, my coachee's 10k was 6.45 yesterday. If she has the same transition as me, 13secs extra would see a 6.58 time, which would be 1:31:15, so pb form, but not close enough to the sub 90.

So work to be done, but hopefully the 3 phrase plan below, will get us closer.

1) Base working MP and MP progressions, steady-hills and 10k "easy" sessions, ie short reps, decent recoveries

2)faster work short period focusing on more V02 max stuff

3) Long build up of HM specific work.