What is the hardest swim set you have ever done?

Repeat 400 IMs!

It wasn't a "set," but if it's open to my interpretation, my hardest swim was a non-stop 2,000 yard butterfly.  

As an age grouper: Three sets come to mind.

• Full 2.5 hour Saturday SCY practice all fly after losing a stupid bet with my coach in the weight room (it's been nearly 20 years since then and I don't remember the exact details of the bet anymore, but I think it was something like I bet him I could do X number of dumbell curls with a 20 lb dumbell). My coach let me put fins on after about 45 minutes, but it was still somewhere around 9000 yards all fly. It was the #1 most painful set/practice I've ever done.
• SCY 100s FR on 1:10 to failure. We did this as a benchmark set in the fall and spring every season and the lactic acid would hit you like a sledgehammer.
• King of the Mountain--Start off with a SCY 100 on 2:00, then 1:59, 1:58, and so on. Drop :01 each time and when you miss the interval, drop down to 75s, then 50s, then 25s. We did it a couple times per season for a 2.5 hour Saturday practice and the sheer amount of lactic acid built up by the end was excruciating.

As a masters swimmer: There are a few candidates.

• A set we did last month that ended up with a total of 3300 yards kick, of which 2500 was with fins. It was a mix of 300s and 400s kick with some swim or pull in between. That's the first set in years where I just flat out bonked. I crashed with about 30 minutes left of the practice and that last half hour was excruciating--no energy and my quads and hamstrings were locking up like crazy.
• A set of 8x200 SCY FR off the blocks on 5:00 that I did about a decade ago, if memory serves it was 1 straight 200 then 3 broken 200s, twice through. What made it brutal was that I was just a few months into my swimming comeback after an extended hiatus and the pool  temperature was in the upper 80s. We'd get out of the pool and lay on the deck in between 200s and the coach would pour water on us.
• 8x150 SCY BK with fins on ~1:30 rest, trying to make it underwater to the far flags off of each wall; speed didn't matter, just the underwater distance. That was back in 2013, I think, and I spent that season training for just the 100 and 200 back. I mostly swam on my own that season and that was a solo set I did. Major, major oxygen deprivation, but excellent conditioning for both of those races. Out of all the sets mentioned, besides the 9000 fly from about 20 years ago, this is the only one I would not do again.

wow

wow

It is great that you have two separate swim lives. Kids and Masters. How long did you take off in between? 

I recall Jim Thornton relating a tale of a masters team-mate doing 11x400 IM in an hour. That a 5:27 interval. The most I have ever done in a single workout is 4, but never as repeats.

I was mostly out of the water for about 6 years. I went to a college that didn't have a varsity or club team, so I pretty much stopped swimming cold turkey, then only got back in the water with masters when I was working a high stress job after college and started packing on the weight.

Holy moly.

It can be tough to get back going again. Glad to hear you made it back to the pool!