Calculate your maximum heart rate by subtracting your age from 220. So if you are 30 your maximum heart rate would be 190.
Keri-anne Payne is a double open water world champion, triple Olympian and Olympic silver medallist in the 10k open water marathon at the Beijing Olympics in 2008. She created the Straight-Line Swimming methodology with her husband, triple Olympian David Carry. straightlineswimming.com