I've been swimming 4-5 miles 2-3 times a week, in a pool, for the last year or so, and occasionally (when I have the time) go to 6 miles easily. I'm self taught (youtube "effortless swimming" is my favorite). I just love to swim, and it turns out, that it's not that big of a deal. Really nothing like a marathon. If I had to compare it to running, maybe closer to a very well trained runner doing an easy 15-18 miles. The secret ingredient is that you have to love it. If it's a drag, and you get bored easy, then forget it, you're beat before you even start. I have no idea what the accepted way of doing it is. But I just started going longer every session. It took about a year to build up to swimming for 3 hours. I find it more mental than physical, that's a lot of time with yourself and nothing but bubbles. For awhile I used a pool radio... but gave up on that, after going through three of them, and endless fights with broken wires and leaky earbuds. Giving up the radio was like being set free, and have never really missed it. I never swim on consecutive days, as my arms would fall off. However, recovery from swimming is amazingly fast compared to running (it's more like cycling) and I'm always ready to go again every other day. Fuel is also a big deal. I always have a bottle of Hammer Perpeteum and a sleeve of Bloks going at poolside. I don't race, other then the normal triathlons. That's probably why it's still fun for me.
Athlinks / Strava