Now we are talking about "heart zone training". It is kind of difficult to describe just in a few sentences. I bought this book 8 years ago (1st edition) when I started wearing my heart rate monitor. It is easy to read and understand. I strongly recommend it if you are into zone training.
HEART MONITOR TRAINING for the Complete Idiot (2nd edition) by John L. Parker.
"I should have taken note of my heart rate readings more so than I did. I know from experience that running at 160+ bpm, which equates to 90 something percent of max, is not a doable thing for me for the marathon distance. Though in training I've run at similar pace and the heart rate would be less that 160, around the low 150's."
Another thing to consider. If you run four 5-minutes runs at an interval pace (faster than your race pace), and since it takes 2 minutes for your body to reach V02max in each run, then of those 20 minutes of hard running you have ACTUALLY accumulated 12minutes at V02max even if you completely recover (HR below 100) between each of the 5 minutes run. But on the other hand, if you run seven 3-minutes runs and of that 21 minutes of hard running you ONLY accumulate 7 minutes at V02max for the time you've spent on running hard. Am I confusing yet?
That said, if you run 20 one-minute runs at the same interval pace and recover fully between each of the runs, then you would accumulate "zero" time at V02max.
So with shorter workouts, you also have to shorten recovery time to allow your body to reach V02max within the workout.
Me? I hardly do any 5/10K races nowadays and mostly concentrate on running marathons. I used to do 1000m intervals but now I am doing 1 mile (1600m) intervals and rest for 90 to 120 seconds (it takes this long for my HR go back down below 120).
Hope all this helps and it wasn't too technical for ya.