So I decided to have a go at the 10-20-30 method and downloaded the app from App Store. I think I found it in one of the pinned posts for JuJu's magic run.
Not wishing to overdo it first time I chose three intervals instead of five and three reps of the minute cycle within the interval (instead of five).
For those who don't know an interval of a minute consists of 30 seconds at easy jog pace followed by 20 seconds at fast-is running followed by 10 seconds sprint (or as flat out as you dare). It then goes straight back to jog, unless you're at the last rep in the interval, at which point you "pause".
I am unclear what it meant by "pause" - it could mean stop completely, or walk briskly or light jog. In the first gap I walked for half of it - till the heart rate got to below 130, then did light jog leading into the next set. Between the second and third sets I walked. Then for the "cool down" I walked, and after that completed the 4k at roughly my comfortable jog pace.
Well it certainly works up a sweat!!
Again I noticed the strange phenomenon that my heart rate went up to 160-163 pretty much at the start though I wasn't exerting myself greatly, or indeed felt breathless. But during cycles 2 and 3 it didn't go up as high as at the start - though I felt more breathless in the recovery period.
Anyway it produced some pretty interesting graphs, and you can see where the fast intervals occurred.
Definitely one to work on and build up gradually - aim for 5 sets of 5 intervals.
I still don't know if the "pause" between sets is supposed to be stopping completely. I think I'd worry that another runner might come by and think I was in trouble!