Hi Tim, I think your performance sounds great. A couple of weeks ago I did a 1500m in the pool in 34:27, not racing, just trying to do it. Now in the lake I have been really struggling to swim anything over 200m or so. So your brief acclimatization in the lake and your time in a shoal of others sounds excellent.
FWIW for myself, I think the almost permanent darkness caused by the murky water combined with the potential endlessness of it all, has a psychological impact on me. So today I used a TT. I set it to a leisurely 1.40, and found it really helped a lot. It gave the equivalent of the pool end. I could focus on the beat and do "just another beat" sort of a thing. That ansd my Garmin set to alarm at 50m intervals distracted me enough so that things began to settle down. I found I was swimming 400m and 500m intervals, separated by floating rests, in quite a relaxed way. Still nothing like your 1.2 mile but still a vast improvement - I enjoyed it (despite nearly getting run over by a power boat!).
A psychological disorder is: "Any personal construction which is used repeatedly in spite of consistent invalidation."
~ George Kelly
"The water is your friend.....you don't have to fight with water, just share the same spirit as the water, and it will help you move."
~ Aleksandr Popov