Does basing intervals on distance amount to poor/confusing training advice?

10x400 is a different workout for a 4 minute miler than it is for a 5 minute miler.

But if you switch it to 10x60s then is the 5 minute miler as prepared for the distance as the 4 minute miler? The 5 minute miler has only done 3k of quality while the 4 minute miler has done 4k.

I think the reason we use distances is because the races we run are distance based not time based.

Anecdotally I've found that I can do about 20 minutes of speed in a workout and about 30 minutes of stamina and if I do much more than that I'm really fried. For speed I can do 400s to 1600s but adjust the quantity so that I get about 20 minutes total at speed. For stamina I can do a 30 tempo at threshold, or tempo intervals totaling around 30 minutes.

The consequence is that I've shortened some interval workouts based on distance. A plan I was using called for 3-5 x mile, but I modified it to 4 x 1200 because I wanted a little faster pace than 3 x mile would have given me. I modded a 3 x 2 mile workout to 3 x 12 minutes.

And so on. I think you make a good point.

I agree with you, particularly as to vo2max oriented workouts like 800s, but the issue you raise can be addressed to a large extent by the duration and nature of the recovery period.

If my wife does 6 x 800, she will run them at 3:25 pace or so, and typically take close to 3:00 of rest, usually in the form of jogging in between intervals, but sometimes just walking towards the end of the workout.

In the few instances when I do that workout (at the beginning or end of a cycle), I would do them at 2:30 or so, and my rest would be a 400 jog (maybe 1:45-1:50). As a result of the shorter rest, and that rest being active rest, my heart rate would start higher than it would if I took a full 2:30 rest or allowed much of that rest to be passive rest. Also, as a slightly older athlete (45), my heart rate climbs up more quickly during intervals than does the heart rate of younger athletes.

So distance of intervals is really only one part of a matrix of factors that you have to weigh and adjust.

For running repeats, distance, pace/effort, and rest interval all have to be taken into consideration to get the wanted results. It doesn't matter if you base the intervals on distance or time.

You cannot just focus on one aspect and call it confusing. All aspects are confusing if you just focus on one.

For one thing, this is accounted for in the design of a workout. When my coach assigns an 8X1000m threshold workout, he is essentially assigning 8X(3 minutes at threshold pace, 1 minute rest). Presumably, the workout would be tweaked somewhat if he was working with athletes who were much slower or much faster (I don't think he'd assign the same workout to a middle school team that he'd assign to a college team).

Also, you have to consider that the race itself takes longer for a slower runner. If you're thinking in terms of time, a 5 minute miler is training for a race that is 25% longer than a 4 minute miler. It makes sense to run longer repeats in preparation for a longer race. That 5 minute miler has to be prepared to run at mile race pace for a full minute longer than the 4 minute miler.