The Running Formula - strides (ST)

I'm using the running formula and in the white plan it mentions "strides". These are "light, quick runs of short duration, with full recovery between the individual strides" and "Strides (ST) are light, quick 20 sec runs (not sprinting) and take 60 seconds to rest between each one" according to the book. I find this incredibly vague and it doesn't tell me anything about what a stride actually is. My easy runs are "light", so should I stride be an easy run that I do for 20 seconds and then take 60 seconds to recover from my barely elevated heart rate?

The thing about "it's not sprinting" implies that it's nearly sprinting. I can run at 8mph while maintaining my cadence but that's not a light run. It also take about 80 seconds for my HR to go into zone 5 while doing that. Is that a "stride"? What HR should I expect to be pushed into while doing it?

I've looked ahead and the next plan (red) has me running 1 mile at threshold level, 3 times, with only a minute inbetween each one to rest, so I assume the strides are supposed to prepare me for that massive increase in exertion (I don't feel it's possible for me to do that at my current fitness level) and should therefore be pretty hard. I just wish it was more clear, this is like trying to interpret Shakespeare

Can anyone explain clearly to me what a stride is? How fast should it be? What should my heart rate be like?