Jack Daniels defines two versions of these. They are meant to increase your Lactate Threshold.
His primary suggestion is to run at a prescribed pace for 20 minutes. His alternative is to do intervals at the same pace, with 30 seconds rest. Apparently according to him, the rest period still maintains the body in a state which works this body system, any longer rest and it falls out of this state.
Jack's paces are very specific, and are based on race times on flat level surfaces. His workouts presume you are doing the same, ie do them on a track ot level road. Basically if you vary from the pace prescribed, then he feels you are doing "junk". If you want to do this stuff, you really must get his book, and do some road and track races to determine your condition.
I have tried to translate these paces into heartrates, which is prescribed by others who believe in HR training. Jack talks about this briefly in his book, but as he is a track coach, I don't think he bothers with it.
For us though, as some have suggested, track running is not directly related to what we do.
For example, some juniors have been subjected to my Threshold interval workout with map memory. During the 30 second rest you memorize the next leg on the O map with course you carry with you, and play it back and try to map read your way along during the interval
Beyond that, in the last few years I have done trail runs at specific HR which is the equivelant of the T pace. And I have extended those beyond the 20 minutes to better simulate the time on an O course. I don't know how long I could go at that pace, but obviously if Vlad thinks it's O pace, then he believes it could go on for an hour or more.
Interestingly, I (that's a capital i) pace in Daniels book is to build your VOmax. He suggests, i think, that this is close to your 3k race pace. But I have run this in a 5 miler (33 minutes) so maybe I have my paces figured wrong, but I was using an HR during the race and kept it at 160, my max is about 170 these days (yeah I know, when I was 40 it was 200, you'll see).
So I'm not sure about the supposed duration of these, but just making myself work that hard helps I guess..............
Glad to see someone else with a problem with the MHR formula. I'm still at least 20 over the formula but haven't done a real max test yet. being urged to go see Docs for stress test to get real value. My experience with the docs is they can't deal with people who exercise.
Your max HR is easily measured experimentally with a HRM. Run for 2 minutes on a flat surface, at about 3K race pace, then run uphill as fast and as long as you can. Measure the HR. It will be your max HR.
I have accumulated 4 years' worth of HR data, and I believe Daniels. HR-based "exercise zones" are a great big lie, propagated mainly by HRM manufacturers. For example, I did a T session on 05FEB2003. I maintained a pace that I'd be comfortable with for an hour. The HR was 175–180. Two weeks before that, I was running as fast as I could through the woods. I could barely get up to 170, and was dead-tired after 35 minutes.
The HR is affected too much by factors other than exercise intensity to make it a useful characterize-all number. The outside air temperature is the primary culprit, but there are others. My educated guess is that the amplitude of motion and adrenaline blood concentration have much to do with it as well.
Imdeed, anecdotal evidence points to Formula One racers maintaining 180–190 bpm for most of the event. Their actual motion (relative to the car) is near-zero, so here's one extreme example to support the adrenaline theory.
It turns out that your leg muscles aid blood pumping through your system to a significant extent (that's one reason they don't recommend stopping abruptly after exercise to people with a history of heart trouble—stopping would present too much of a load to the heart). Thus if you orienteer (large range of leg and mid-body motion), you may need a smaller HR to maintain the same blood throughput/VO2 rate compared to flat track running.