I had 6 weeks off over Xmas with flu that wouldn't FO. My first swim back as a disaster, was way off pace. It wasn't just fitness but technique was all over the place. Within a couple of weeks it had come back a fair bit but still some way from "peak".

Your event doesn't give you long to build back up at all. But downstream in a wetsuit, take it easy an accept your swim time will be poor. Not much else you can do. Don't try cramming in a load of training, all that will do is add a load of fatigue.

I'm not a fast swimmer, but I took almost a year off. First swim back in the pool was a disaster. Shoulders, lats, tris all were burning at 200y.

Within about 6 workouts, building up from 1200, I'm back to being able to swim a continuous mile at race pace (about 1:45/100y). I had much more in me, but it was time to go to work.

Plus, my workouts got much faster... I can now hold sub 1:30 for 10x100y sets on :15 rest. Something I could NEVER do before, even at peak fitness.

YMMV

I was off 6 weeks due to injury and travel. It took two weeks of swimming 5x week and I was back to where I left off.

After a break my coach had us to this set for a full week:

600s divided into 4 consecutive 150s where you build every 150. Unlike a traditional build where you tend to focus on the fast part when you do this you focus on the easy part. Maybe only 15 seconds of fast swimming per 150. The challenge was to get right back into the easy groove after the 15 seconds without stopping.

This was a US squad made up of college swimmers so we would do a lot of them. As many as 10 in one workout with that being the entirety of the workout. You could consider doing 4 consecutive 75s or 100s instead but based off the same principals. I have concluded that this approach is a great way to get the feel for the water back.

Keep at it. It will be tight, but you should be able to be back in some semblance of swim shape by May 20. It would probably take 4-6 weeks to recover all your lost swim fitness, but you don't need full fitness to survive a down-stream, wetsuit-legal swim leg. And with a wetsuit and downstream, there's no reason to press the threshold if you're unsure of your fitness. You just need to find something of a comfortable pace that you can manage to hold. Better, in this case, to leave yourself a little safety margin than to try to swim right at the threshold.

I'm normally not an advocate of doing much pull buoy work, but if you continue to have cramp issues (I'm assuming you're legs are cramping) after your second or third workout, I would go exclusively to the pull buoy to get ready for this upcoming race since you know it's going to be wetsuit legal.

Really, there's no reason to blow up. Downstream, wetsuit legal, you can float to T1 if needed.

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"They're made of latex, not nitroglycerin"

Great advice! I think I might try the 75, 100 or 150 repeats. I realize that it just isn’t going to come back immediately. My first 100 was 1:32. I felt pretty solid about it. Then I dropped between 1:39-1:41 through the rest and cramped at the end. I guess I’m just going to have to ease back into things gradually. Perhaps I’ll survive the 70.3 swim by going easier to save myself for the bike and run. Better than blowing up. I think I can manage with a wetsuit legal downstream swim.

Well, since it's probably Chattanooga, you do have 400m upstream before the turnaround. ;)

I would try to ramp up to a main set of 5x400, 10x200, or 20x100 since you've getting to the specificity stage of your training. Even going easy will take something out of you if you haven't practiced much for the distance.

I would imagine that the answer depends on your swim background, how long you were off, and your age. I am 60 and took January off this year due to a nagging shoulder problem that wasn't going away. Most of the literature seemed to say I would be back in a couple of weeks. It took two months before my times were back to where I was when I stopped.