Swimming, 7/19

Now I'm swimming 3km every time and tried 3.5km but it made me tired.

Here is what I do:
* each lap is 50m, and 10 laps makes one set of 500m

500m kicking only, on my back (warm-up for backstroke)
500m backstroke
500m backstroke/freestyle (25m backstroke, 25m free)
1 - 2 minute rest
500m backstroke/freestyle (25m backstroke, 25m free)
500m freestyle
500m kicking only, swimming on my side to practice balance

For 3.5km, plus ...
500m freestyle, after kicking

2 minute rest and walk a little to cool down + stretching

My main purpose is to build strength and stamina, not speed. So I swim at a medium pace and I try to finish one set in less than 15 minutes. I do 3 sets (total 1.5km) with no break, rest, then do another 3 or 4 sets.