Two words "Sweet Spot"
I've only been swimming for a couple of months and am otherwise like you. Exercise time= Out doors time, and so I have been training to be in open water, which is very intimidating.
The focus on balance and rhythm in TI swimming has been the answer for me. Treading water is not resting. In fact I find it quite tiring. Being nicely balanced in the TI sweet spot is far easier to sustain. More importantly it lets me catch my breath and think through where I am at and what I am doing. I can pace myself and think about what I am doing.
I prefer to breath every three strokes so that I am balanced and using both sides equally. I can't always sustain this, and so I will often do sets of strokes 2 or 3 sets of strokes breathing on one side and then switch and do the same on the other side. I've found I can sustain this rhythm and many variations of it for a solid hour in the pool without stopping (as long as my damn googles don't fog). TI also taught me to take get my whole body into my swimming and really leverage my core instead of just my wimpy little arms. That is a great advantage for someone used to doing distances on land.
If you have every done X-country skiing, especially skate skiiing you will quickly recognize the TI Cadence in your body. It is a different body motion and timing, but the same sensation. Explosion-glide Explosion-glide Explosion-glide And then you add in some breathing and figure out how strong an explosion you can sustain.