The concept of CV paced intervals is new to me and I am intrigued by it. I notice that the most common distances for these intervals seem to be either 1k or 1200 meters. As a follow-up question, how do you feel about using CV pace for 1 mile repeats every other week, with the 1k or 1200 meter distance on alternating weeks?
This question comes up quite a bit from those new to the "Tinman way", so I have kept the below excerpt easily accessible. Probably Tinman's most thorough explanation for why 1k reps are a 'sweet spot' for CV work.....
The Tinman method of training uses repeat 1km CV reps as a primary baseline fitness tool. It gets you in shape to do specific race-pace training. I've found that doing longer CV reps isn't really going to do much more in terms of baseline fitness than doing 1km reps. That doesn't mean the 1200s, 1500s, 1600, or 2k reps can't be used - for variety: It means that noticeable improvements in baseline fitness appear to be similar throughout the 1k to 2k range.
What counts is running at CV pace, running a particular volumef of CV, and controlling the recoveries between reps (creating specific training density stimulus). Let me use and example. Jon, 31 minute 10k runner, may run 8 x 1km at CV pace, jog 1 minute between reps or 4 x 2km at CV pace, jog 2 minutes between reps. The overall stimulus is about the same - negligible.
Now, from a pragmatic point of view, if Jon runs 1k reps and finds that he isn't doing well during the workout (he's extra tired, ill, or whatever) it is more likely, experience shows, that he can call it quits and not do much harm - because it is just 1k (3-4 minutes of running). If Jon is running 2k reps, it is a bigger commitment workout, and he is likely to "at least finish the rep." The problem is that particular rep - which is fairly long - can put him in the "hole." The result is not pretty! He experiences a setback that lasts a few days.
Am I saying Jon can't do longer CV reps? Not at all! I am just saying, as a coach, I tend to prescribe 1k CV reps because of three main reasons:
1) They are mentall "doable." They aren' so long that you have to psyche yourself up to do them. Also, as a coach, you can find a nice 1k loop at many parks or on school grounds and monitor 1k reps visually. That's a bonus! You can see when somebody is straining or racing when they should be relaxed and training, instead;
2) If your pacing is awry (say, you've gone too fast) on a rep, the distance of going too fast is not that long, so you won't really have to keep running a long way at the wrong pace. You adjust on the next rep and the workout becomes a success. Mission accomplished!
3) If you are having an "off" day and decide to bail after you complete a rep that isn't going well, you really haven't caused any harm. On the other hand, if the rep is long and you want to finish the rep (a common thing for runners to do), then it is a problem - you've gone to far feeling awful!
I will give longer CV reps to runners who are quite fit, experienced, and having a streak of consistent training. If I know they are stable, then longer CV reps may be prescribed. If someone isn't stable, has been struggling here and there, then I will NOT give them CV reps that are longer than 1k.