Please clarify the above set. Were you doing 16x50 breast on a 50-second interval and holding 42 seconds at a 70 percent effort? That's Olympic quality. I had to do sets like that in college, and that was at a top-level university! I don't think I did 16 of them, though.
Also, keep in mind that you took six years off and have only been back for three months. Your body is trying to find its swimming shape again.
If you are doing 16x50 on :50 now and holding 46 seconds, you are still quite an animal. I don't think any of the revered distance (i.e. 200) breaststrokers in Masters swimming can do that set.