Just to get the ball rolling here, I'd like to post a practice we did last night. It was written by one of our team's co-coaches, Bill White, who swam at the University of Louisville and is the most knowledgable swim coach I have personally had the pleasure to meet (this includes, by the way, one year at the U. Michigan.)
Anyhow, Bill's workout seemed impossibly challenging, but somehow he coaxed us into making it.
5 x 200 on 2:45 warm up
5 x 200 on 2:35
4 x 200 on 2:30
3 x 200 on 2:25
2 x 200 on 2: 20
1 x 100 all out sprint
1 x 100 cool down.
We got about a minute and a half rest between each of the sets above. Our team only gets the pool for 60 minutes, so the total practice was 4000 yards completed in 60 minutes. I suspect this practice would be pretty easy for younger type swimmers, but I am 49 and it was quite challenging. I should point out, however, that when it was over, all of us who made it felt a tremendous sense of satisfaction!