Sounds a bit toppy to me too. If it's realistic goal pace, 8 miles in training might be just about doable as a time-trial but would leave you pretty spent, whereas I think a tempo run ought to be tough, but possible to complete under control, feeling like you could squeeze out another mile if really necessary, so for me something like 6M @ HMP would be my limit.
I'm a big fan of mixing up different tempo paces working around target pace, so anything from 3-4M @ LT up to 10M @ MP, or some sort of combination, e.g. on a 3 mile loop I like to do a 6M continuous progressive tempo run, with one lap @ MP then another @ HMP, possibly building towards LT or even 10k pace depending on how good you feel. If you cycle through 3 or 4 different weekly tempo runs you can check on progress by repeating a session monthly or so. Keeps things fresh and it's nice to see progress without having to beat your times on the same run every week.
Edited: 18/02/2013 at 21:33