Running a "tempo" run too hard, do you still get some benefits or does it set you back?

The answer to your question depends to a great extent on what racing distance you were trying to prepare yourself for, and how far away that goal race is.

If you're preparing for a goal half marathon, for example, and it's not until December, then you didn't really help yourself at all. You should be doing longer tempo runs (4-6 miles) at current half marathon pace. Your tempo run didn't work the energy system that you're trying to develop.

If you're training for a 5K that happens three weeks from now, on the other hand, you probably helped yourself--although a tempo run in which your legs actually hurt is never optimal. Nor is a VO2 workout in which your legs hurt. Take a couple of easy days and let those legs heal.

It depends on where you are at in your racing/training cycle. If you are in a base period, far out from any major races, then letting it rip a bit more on a tempo probably won't hurt you, as long as you do it infrequently (once a month?). If you are trying to do training that includes 3 quality days per week and or a fair amount of racing, then going too hard on the tempo is counterproductive.

At paces faster than tempo, you still get the benefits of tempo, you just can't do it as long (as you found out the other day). The benefit of running at the correct pace is that you can go for a longer duration and still be able to recover for the rest of the week's training.

I do a lot of cruise or tempo intervals with my athletes early in the season. It is still really hot here (supposed to hit 101 degrees today), so a sustained tempo run is impractical. In my early years of coaching, I used to let my kids paces creep faster and faster during the workout, thinking, "If they can do the workout, then they should do the workout. It will only make them stronger". Those teams had some success but were inconsistant. Over the last couple of years, I have very strictly controlled the paces on these workouts, opting for slightly higher volumes and lower intensity. Our team has been able to perform more consistently during the season and "peak" better at the end lately. I attribute this at least partly to the more strict "speed limits" I set during our workouts.

Dave Wottle wrote:

Wellnow, maybe this is for another thread or maybe not, but could you elaborate on your claims about lactate production. I was under the impression that(unbelievably oversimplified) during glycolysis, glycogen is broken down into pyruvic acid and that without oxygen pyruvic acid is turned into "lactic acid."

That is the pathway proposed by Meyerhoff in 1921.

More recent biochemichal analysis proposes that muscle glycogen is broken down (glycogenolysis) into pyruvate then lactate. This last step regenerates NAD+ which is needed to produce more pyruvate.

Glycolysis is the brekdown of blood glucose, which itself is derived from liver glycogen. Glycogenolysis and glycolysis differ significantly in their production of anaerobic ATP supply. Glycogenolysis produces less H+ and six times as much ATP as glycolysis.

To summarize, all stored carbohydrates must be converted to lactate and reconverted to pyruvate before further conversions produce ATP by aerobic respiration.

Neither pyruvic acid nor lactic acid are produced, but pyruvate and lactate.

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Yes indeed it would be a disadvantage for a Marathon runner to compete in a flat out 800m race at any time during his Marathon race.