any input on expanding lungs

You don't need to build up your lungs. There have been Olympians in various sports with only one lung. It's not the volume of air you can breathe in that's important--it's the amount of oxygen your blood via hemoglobin can extract, how fast this can then be circulated to your tissues, and then how good your muscles have become adapted to extracting the oxygen for use. Training at altitude can increase your red blood cell counts and hemoglobin levels; I suppose (but don't know for sure) that hypoxic sets where you hold your breath while training might have a similar effect, though it can be a bit dangerous, and I have heard conflicting opinions on how useful this actually is. Training hard, however, will get the muscle fibers used in swimming to produce what are, in essence, additional oxygen extractors. One other thing: you might not be inhaling deeply in the first place, or perhaps you're not emptying your lungs sufficiently to blow out the stale air so more fresh air can replace it. Oh, and the last item: it's the build-up of CO2, not the depletion of O2, that drives that horrible hunger for air during a breath hold. I once did an experiment where I held my breath for 1 min 30 seconds, and using a pulse oximeter, got my blood ox down to 94%. (Normal at sea level is 99 to 100 percent; I was at Boulder, CO, where normal is 98 percent.) I felt really, really starved for air. Then I entered this engineer's hypoxic chamber, and went up to a simulated base camp at Mr. Everest altitude--18,000 feet. I could breathe in and out at will, blowing off CO2 before it could accumulate in my tissues. I may have felt slightly light headed, but I didn't feel the slightest bit starved for air. The pulse oximeter at this point measured my ox level at 85 percent-- enough, the engineer assured me, to put me in an ICU if I showed up in such a state at an ER. Training can help you tolerate the feeling of built up CO2 as well, as those free divers who can hold their breathes for 7 minutes plus clearly show.

You don't need to build up your lungs. There have been Olympians in various sports with only one lung. It's not the volume of air you can breathe in that's important--it's the amount of oxygen your blood via hemoglobin can extract, how fast this can then be circulated to your tissues, and then how good your muscles have become adapted to extracting the oxygen for use. Training at altitude can increase your red blood cell counts and hemoglobin levels; I suppose (but don't know for sure) that hypoxic sets where you hold your breath while training might have a similar effect, though it can be a bit dangerous, and I have heard conflicting opinions on how useful this actually is. Training hard, however, will get the muscle fibers used in swimming to produce what are, in essence, additional oxygen extractors. One other thing: you might not be inhaling deeply in the first place, or perhaps you're not emptying your lungs sufficiently to blow out the stale air so more fresh air can replace it. Oh, and the last item: it's the build-up of CO2, not the depletion of O2, that drives that horrible hunger for air during a breath hold. I once did an experiment where I held my breath for 1 min 30 seconds, and using a pulse oximeter, got my blood ox down to 94%. (Normal at sea level is 99 to 100 percent; I was at Boulder, CO, where normal is 98 percent.) I felt really, really starved for air. Then I entered this engineer's hypoxic chamber, and went up to a simulated base camp at Mr. Everest altitude--18,000 feet. I could breathe in and out at will, blowing off CO2 before it could accumulate in my tissues. I may have felt slightly light headed, but I didn't feel the slightest bit starved for air. The pulse oximeter at this point measured my ox level at 85 percent-- enough, the engineer assured me, to put me in an ICU if I showed up in such a state at an ER. Training can help you tolerate the feeling of built up CO2 as well, as those free divers who can hold their breathes for 7 minutes plus clearly show.