When running tempo workouts, which is more important: speed or distance? So, is it better to do a 4-5 mile tempo at closer to 10K pace or a 7-8 mile tempo at closer to HM pace? This is in the context of training for a marathon, so I assume longer would be better, but I am interested in thoughts on this
Both. I prefer a progression throughout the training cycle from shorter faster tempos to longer efforts moving closer to MP later in training.
IMO both of those efforts are too hard
4-5mi at HMP and 6-10 at MP - HMP are reasonable
To answer your question, the longer the better. In marathon training, I wouldn't cap your tempo distance at 8 miles.