The main reason for doing tempo runs at a carefully controlled pace or heart rate, i.e., just on the friendly side of your lactate threshold (the intensity at which you begin to accumulate lactate more quickly than your body can clear it from your system) is that you derive ample benefits without knocking yourself silly and requiring several days to recover, as you would after a race or gut-busting hill or interval session. It is not, as many seem to have been led to believe, that you somehow spoil the session by going too fast, negating all of the benefits of the run. Hard running, however, isn’t The Price is Right, as you still stimulate gains even by going over your physiological “bid.”
A time trial, on the other hand, provides both a psychological and a physical dress rehearsal for a race at a particular distance. Some track athletes do these over their full race distance at a pace that is necessarily a little slower than whatever their race pace might be on the same day (almost everyone benefits incrementally from the adrenalin and focus a competitive situation entails). More commonly, runners perform all-out efforts over a distance about two-thirds that of their chosen event. A 1,500-meter athlete looking to run 4:30 would aim for a 1,000-meter “dry run” in three minutes; a marathoner aiming to crack three hours might do a 15-miler at just under 7:00 pace.
A workout that includes both threshold-effort running and race-pace running, then, can be assumed to provide a mixture of both: an improved ability to run at, as well as recognize, the all-important “red line” between aerobic and anaerobic intensity, and the oomph of running close to all-out so as to reinforce and enhance race-pace turnover and anaerobic power – what some call “speed endurance.”