My college coach was big on Jon Urbanchek's colored cards system- do a T-30 steady swim at the beginning of the season to find aerobic threshold pace, and maybe once or twice more during the season. Then you got out the different colored pace cards and were supposed to use those to find out what your goal time should be for every rep of every set based on what kind of effort (aerobic, anaerobic, VO Max, all out sprint) you were supposed to be doing. As someone on the USMS board recently said, you come to fear the purple cards because those tended to be the 'till you're gonna puke' deals.
Swimming is almost entirely intervals because your muscles give out before your aerobic engine does. You don't want ot re-enforce bad technique, and even 5-10 seconds after a 200 yard/meter rep (taking 2-2:30) can seriously help reduce muscle fatigue to a point where you can kind of hold technique together without causing too much of a break for aerobic purposes.
My typical afternoon practice would be something like:
1000 yard warm-up- usually your choice of how you did it, but sometimes some light psrinting in the last 200 or so
2100 yard main set A- something like 7x300 aerobic hard with about 20 seconds of rest between each of you were making the interval right
1000 yards of kicking- ex- 10x100 on 1:50 to give you about 10 seconds or rest between each. Keep an aerobic pace, but still be able to carry on a conversation with the person in the next lane
2000 yard main set B- 16x125 swum 75 freestyle-50 other stroke with about 10 seconds est between each
10x50 sprint- 0n 45 seconds, hold in the 33-34 second range for each
A couple hundred warm down
The infamous fish burn set-
1000 warm-up
main set of
5x100 on 1:30
4x200 on 2:30
3x300 on 3:30
2x400 on 4:30
1x500 on 5:30
so you pretty much increased intensity and distance per rep as you went on. In my lane of mediocrity, if you made it midway through the 300s, you were a studette. Once you failed in the set, you got out, did 100 push-ups and 100 sit-ups, and then swam continuously until someone finished the set or everyone failed. And the coach would kill you if you got in the way of someone who was still making the send-off interval.
Lots of fixed send-off intervals in swimming because of the limitations of pool space. The teams I swam on, we'd typically have 6-7 people in a 25 yard lane lane, and if you only had 4, it seemed blissfully uncrowded.
This was for a pretty average D3 team. We'd do another 3-4K for morning practice during the school year, some dryland before practice, and 15K per day long course meters on the post-Christmas trip.