Find your limits. By constantly reducing a rest interval for 50s, 100s, or for any distance, you can create a fun set that starts very easy, and ends up VERY hard.
Create a short set — for example, 3 x 50 — and repeat it many times. Take a short break (about 30 seconds) between the sets to reset the clock, and to recoup and prepare. Each time you start a new set of 3 x 50, you’ll reduce the interval. Try to reduce the intervals in a systematic way, and since it’s very difficult to get 1/2 seconds on the pace clock, use whole numbers.
A set such as this gets tough not only physically but also mentally, because you have to keep track of the intervals. You have to remember WHEN you pushed off… and then think forward to WHEN you have to push off for the NEXT swim. It’s quite a challenge. Here’s an entire set example:
3 x 50 on 1:00
3 x 50 on :57
3 x 50 on :54
3 x 50 on :50
3 x 50 on :47
3 x 50 on :44
3 x 50 on :40
3 x 50 on :37
3 x 50 on :34
3 x 50 on :30
Etc., etc., etc., until you just can’t make it anymore.
One of my Masters swimmers asked the other day, "How far down do we go?" My response was, "Anyone who can make 3 x 50 on the :20 is excused for the rest of the season." We all had a nice laugh at that.
But, seriously, this type of set will teach you how to PREPLAN how far you can go. It also will teach you to take it EASY in the beginning in order not to over-swim the set. The goal is not to make the initial 50s by as much as possible. The goal is to conserve your energy for when you really need it.