Sometimes I feel like I have two speeds for practice - fast and slow. However, we have two "non-slow" days on our schedule, and they're named differently, so that usually indicates to me that they should be run at
different paces. Interval days (or speed workouts) are supposed to be done at a pace that is above your
lactate threshold, while tempo days are done more at your LT level. It's important to actually adhere to these paces, as they actually train two different "energy" systems.
So, which is better? Well, a good training schedule (like the one coach uses) incorporates both. Interval training serves to raise your
lactate-threshold faster, while tempo training teaches you to
sustain this pace for longer periods of time. Personally, I've always found tempo running to be more mentally challenging because you're running right at that somewhat uncomfortable point for much longer. The lack of breathing, increased fatigue, etc. build as time goes on, and this is sometimes harder to capture in a shorter interval. Additionally, I feel that tempo running helps me to "learn a pace" better... if I go out too quick, well, the end is going to really suck. Having said this, leaving out interval training has the effect of "blanket packing" for me... suddenly my 5K pace after the first mile looks like my 10K pace, which begins to approach my half marathon pace after not too long. This makes sense because, as Snell's research points out, tempo training teaches you to run right at your LT level (but doesn't significantly raise it), which is right around 10K to half marathon pace for most runners.
So, the long and short of it is... run your interval days at "interval" pace and your tempo days at "tempo" pace!
