Scooby- those tempo runs do feel pretty tough at least with intervals you get a rest every so often. Well done for still getting a tough workout in.
I had the same run tonight, 9 with 5 a LT and it is tempting to bin it in the first mile or so, just need to get to half way.
I've set up a program on the Garmin of mile repeats (*5) and a HR zone of 88 to 92% of max. I find it easier just to keep focus on the hr band.
Managed an average of 6.21, so pleased with that.
I'm 5'9", some would say I'm a 5'8" short arse....they lie!