Lean, beastly legs are the indicator of fitness. Biceps might help some with paddling, but when you're 20 miles offshore, you need your stabilizers to stay alive. Start lunging.
With an upright posture, thrust your right leg forward and out, so that your right knee creates a 90-degree angle between your calf and your hamstring. Your left knee should just barely touch the sand. Then, bring your left leg up and out, and lower your left knee into a 90-degree angle, so that your right knee now nearly touches the sand. Repeat until you're burned out.
"I'm not carrying weight," Dave says. "I just start lunging my way down the beach. I try and do at least 100 every time. If I've been doing them for a few weeks, I get myself up to about 400."
Kalama also advocates backward lunges. You guessed it: same form, in reverse.