these workouts offer a consistent weekly dose of race type suffering. granted you won't be quite going race pace for the distance of your training session, but you should always be going faster than race pace for the distance you're training for - maybe significantly faster. Tempo sessions also take alot of mental work, and after big races or time off, i've always eased back into them. I'll usually use as a starting point for pacing my best previous effort of some distance (peak pace) - for example, i currently consider my peak pace for running to be 9 miles in an hour (6:40 per mile pace). Whenever i start getting serious about running after a hiatus, i'll look to approach this speed during my first real tempo workout back, even if only for a brief period. A typical 'first' workout might look something like 2 mile warm up (WU), 2 mile tempo (6:40 pace), 3 mile easier (7:30 pace). The next tempo session would build on this one, perhaps having a 1 mile warm up, 2 mile tempo, one mile easier, and 2 miles tempo, and one mile warm down. eventually, the lions share - or even all - of time during a tempo workout will be spent at or near 'peak' pace.
Ultimately, i find that using the three hour a week program i'm pretty much always in good enough shape to reach my previous peak levels in any particular discipline in about 6-8 weeks of focused training (although i don't always have that long between events). Although i'm sure i could get faster, i think that my current 'peak' level is pretty close to my potential (at least as far as running goes) based on this type of program. Perhaps i could improve so that i was running 10 miles at a 6:30 per mile pace or something, but this would likely involve more sustained periods (12 weeks? 16?) with less cross training - and the truth of it is that i typically have some big race or trip planned that is difficult enough that it requires significant recovery afterwords - these races or trips act as sort of a reset button for me. successive endeavors usually involve different disciplines and so at this point i'm pretty content at where my peak levels seem to be.
One key point that remains to be made about tempo workouts is how hard to work during non-tempo intervals. It is important to allow the warm-up to build quickly into a solid effort (Zone 2/3) and to keep all non-tempo intervals squarely in Zone 2 or above. Zone 2 can be thought of as the pace you'd be spending most of your time in if you were to go out and race a marathon at your current fitness. In my sample workouts above, it's clear that 'easier' doesn't mean easy. The work intervals should feel rather difficult, so much so that you're very happy when they are over. by not letting your heart rate drop back into Zone 1 you're physiologically and psychologically teaching yourself to recover at higher levels of effort. The payoff is that over time, Zone 2 will become 'easy' - because this is the slowest pace you ever run in training. During the initial period of time when i applied these principals to my running, not only did i get faster at the top end, but i also got faster at the bottom end, and disproportionately so (my top end mid distance pace went from about 7:40 to 6:40 while my 'slow' mid distance pace went from about 9 minutes per mile to 7:30). In the context of major endurance efforts, this is huge - as your 'easy' pace is where you're going to spend most of your time during an ultra distance race. if you're comfortable physically and mentally in Zone 2, you're going to just move that much faster.