Bumping this slightly old thread to tell my story. Similar to OP in that I'm 50 (51 in a few weeks), 6'1", 220ish ilbs, lost 70+ lbs in the last year or so and decided to do a sprint triathlon with my 18 yr old son in May. I regularly cycle 500+ miles a month in season and run often. I have done two marathons, though they were 19 years ago.
I, naively thought swimming would be no problem due to my cardiovascular fitness biking and running. WRONG! Like the OP I was gassed after my first 25 yards. I've swum since I was a little kid but never more than a lap or two. I started six weeks ago doing 200 yd workouts. I recently signed up for a group swim class with a coach who focuses on swim technique. This morning I did 900 yds of a structured workout with a 150 as my longest swim. I know I could do more distance now but will work up to it slowly.
After just two stroke lessons I feel like a different swimmer. I can't emphasize enough how much difference that has made. A more efficient stroke propels you further with less effort. My son swims on the local swim team and has developed some bad habits in the process. He is struggling with the coaching because his muscle memory is holding him back. Don't tell anyone, but I am faster than him for now. He can swim all day long and would eventually wear me down, but since I was such a blank canvas, I was able to change my stroke fairly easily. So don't wait, get someone to look at you swimming and correct your stroke flaws. It will definitely pay off.
My coach also has me trying different breathing patterns. Some laps I breathe every five strokes, some every three and some every two. This has definitely helped me build better lung capacity as well.
And I'll echo squirtdad - get a better coach than me!
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