Assuming a short commute to work might be a stretch. My commute into downtown keeps getting longer as they add stoplights along my route, and more people move here. I don't know many people who live downtown due to cost. Unless you're on a super high end budget, you're likely looking at an apartment instead of house, if that matters to you.
The most active cyclists I know either live northeast (Pflugerville) or southwest (Circle C, Bee Cave/Lakeway, and even farther out), but I'm pretty sure there are group rides that leave from downtown. Getting into downtown for an early workout on the weekend-- either a group ride/run or swim at Barton Springs-- is pretty easy from anywhere.
I do know some triathletes who swim with the UT masters program. They like it, and I would guess that's the place to find the fastest (read: ex-college) masters swimmers, but there are other programs. I think most of them are southwest or northwest Austin (Nitro, Circle C, AASA on Southwest Parkway). What's 'best' might be different for everyone (competitive/fast swimmers, developmental coaching, etc). If you're looking for public pools, downtown has the best choice: Barton Springs, Deep Eddy, Big Stacy (
full list of public pools here). Lifetime and the YMCA have several pools spread across the city if you're a member of either of those gyms. I've swam at most of them, and both groups keep their pools clean, but their outdoor pools are hot, whereas their indoor pools are nice and cool, but more crowded.
As far as open water, you can reliably swim in Lake Travis, which is west of town, but you can simulate open water at both Barton Springs (downtown) or the Lifetime Fitness (used to be Pure Austin) on Braker Ln (NW Austin) which has the Quarry Lake behind it. On the east side, Lake Pflugerville and Walter E Long lake both host races and so I think you can swim in those lakes as well just about any time you want.
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