Dean, a 14second range for tempo sounds too big, so I thought i'd look at the numbers!
a 10k of 33.03 gives a 10k pace of 5.19
Therefore, you can see that 5.23 is asking a lot, and no wonder you can't get close to the top end, as it's borderline 10k pace!!
Your 10k current zone would probably be say 5.17-5.22
So too fast is one issue, and maybe too many miles at that pace another.
I'd suggest your "true" tempo/Threshold/10mile race pace is more like 5.32-5.37 flexible on a couple of secs....
McMillan seems to have flaws in both offering up too fast paces at times, as well as suggesting ridiculous short distance times (5k&10k) off 10m and Half marathon
I'd wager if you went and did a session of 2miles at 5.32-5.37, 90secs, then 2miles 5.32-37 you'd come through it feeling worked out but a fairly hard effort.
Edited: 23/01/2013 at 07:55