NeedSwimming Advice!

halips

Hi everyone! I'm an avid swimming (6 times a week-about 2 miles a day). Recently I have been feeling like my workouts are not challenging enough. I'm looking for some advice on burning more calories and increasing my heart rate. Any input would be appreciated!

My Current work out:

200 free warm up
500 free pull
200 kick
100 free sprint
repeat it starting at 500 pull.
end with 200 free cooldown

A little more info:
I swim my mile in about 27 minutes. I only like to swim freestyle. I use flip turns. I breathe every 5-6 strokes. I wear a polar FT4 to monitor heart rate/calories and in about 30 min I can burn 250 cal. My issue is that 30 min with this work out used to burn me 400+ cal. I was slower and less conditioned, so I understand my body is more efficient now but 250 seems low. My average heart rate sits at 134 with the highest being on my sprint at 148. Help!