Best Swimming Excercises to Burn Fat

I've just recently got back into the pool. I really love to swim!. It feels better on my knees and body than running does. Right now I do a muscle pump class twice a week that is mostly with free weights on Tuesday and Thursday, a Tabata class on Saturdays, and something cardio related on Mon and Wed.

I want one of my cardio workouts to always been swimming. I also want to be focused on fat burning vs. muscle building since I feel I'm already getting that with other workouts during the week. Any suggestions on a routine? Right now I do around 1,000 yards with a 400 warmup freestyle and kick board followed by 4-100s between freestyle and breast stroke, followed by a 200 cool down.

Should I go more for long distance or mix in sprints? What has worked for you to burn fat?

Thanks for the advice!