What you wanna do is get yourself some sets, and time them. Also known as on the clock. There are websites out there with tons of good workouts, i believe
http://forums.usms.org/forumdisplay.php?forumid=95 is an excellent forum which people list workouts they are doin, tryin to accomplish, etc. Sets vary greatly, from all out sprints with lots of rest to long distance, or all out sprints with hardly any rest, etc. Depends on what your tryin to do. Dont just swim unorganized, get yourself somethin to do for that workout day and stick to it, dont forget warm up and flushing out and the beginning / end
Chuckles