I learned this run workout back in 1998 while living in Seattle. I initially learned it as 20 X 1 and then extended that to 40 X 1. Canova has similar workouts that employ longer interval lengths which I've run in the past. Like 8 X 1 mile on 400m.
On paper the workout doesn't look like much but the devil is in the details, or in this case, the pace. When I ran the 8 X 1 mile on 400, each mile ended up being in 5:20-5:30 and each 400 was in about 1:30 (6:00 pace). The idea is to run the hard interval at or slightly above threshold and the "easy" interval just below threshold.
The key to the effectiveness, and difficulty, lies completely in the pace of the "easy" interval. When you only ease up slightly when you're hurting it's like moving your hand just an inch further from the candle. It doesn't help much. So there is also a solid mental component to this.
This is a great workout for teaching not only where your edge is but also the skill of making just subtle pace changes to salvage the over-all pace. Just like in a race. You'll never nail this run by using a GPS or strict paces. This has to be done more by feel, running just hard enough and then recovering just enough. If you try it then measure the first attempt. Get your 20:00 distance. Then in the future try to run farther for the 20:00.
Variations that I love for this structure are doing it on a constant incline and also doing it on the bike. I have a 2.5 mile climb near my house that starts at 8400 ft and ends at 9200 ft altitude. Trying to run easy and recover on a 6% grade at altitude is an oxymoron so this ends up being basically 20:00 hard. And really that's the best way to think of this session!
Another variation would be to extend the workout. If you do 20 X 1 on 1 then you want to dial back the paces slightly. And for a really long one like 40 X 1 on 1 you'd run the hard minutes at threshold and then ease back to Z3 (tempo or marathon pace). So you could think of it as 1:00 at 10k pace on 1:00 at marathon pace.