2/3 rds swimming

I'm getting into a routine swimming once a week. The solution to the routine has been to swim at what I call two thirds space. So, instead of 3000 metres in an hour the mileage is nearer 2000. But I keep to a structure of a warm up, kick and drills before a butterfly set and then some hundreds. I should keeping a better track of my heart beats too, I found this post via the mepath swimming activity stream. I've not know the full science about heart rate and training so this was a useful find.