Endurance

Pacing is tricky.  I tend to err on the side of caution with pacing and often feel like I finish with a bit too much left in the tank.
For building endurance I like pyramid sets (say 50,100, 150, 200, 250, 300, 350 and down again) with short rest intervals (15 to 20 seconds max).
But for confidence it's nice to do a continuous swim every now and then, go through a slightly bad patch, recover from it and have the confidence in yourself that you can recover actively while swimming after a perod of over exertion.  
​To do so, back off, change gear, use less pressure in the catch, relax and focus on swimming as easily as you can, maybe at a slightly higher cadence to get a bit more breath.
Then get back into the groove after a few minutes, start focussing on weight shiftng and swim within yourself.