By Craig Siegel and his coach Tom Kristek Craig (on the left) is a former NC Masters swimmer, and former team mate at TRYM. He now resides in Austin Texas. He sent me this endurance workout, courtesy of his current NITRO coach Tom Kristek (on the right).

The goal on the main set is to maintain the same pace throughout even though the intervals become tougher. Great endurance workout.

300 swim, 200 kick, 300 pull

Main set

100/200/300/400/500/500/400/300/200/100

On the way up, pick a per-100 base interval that is moderate (example 1:45)
On the way down, per-100 base interval should be 10-15 sec faster (example: 1:30)
Stick to the base interval and take no additional rest between any repeats. Try to swim as fast or faster on all the repeats coming down

100 easy

3900 meters