Workout

Warm-up (500)
500 swim
...every 4th 50 drill

Set (600)
4 x 150 on 2:30
...50 kick/100 swim  

Set (1600)
6 x 150 on 2:15
...descend 1-4
1 min rest
6 x 150 on 2:00
...build to fast by 50's

Set (400) 
8 x 50's on 1:00
...25 underwater
...25 FAST

Cool-down (200)
200 EZ