For pretty much my entire adult life, I'll get into a routine and exercise regularly for 3-9 months. Then, something happens, I get off-schedule and I'll stop (sometimes for just as long!). So I can't say I have found anything that keeps it being consistent long term, but the things that I realize do help me are:

*routine - I'm not good at skipping days. If I tell myself I'm doing it every day (well, every week day), then I'm much better about sticking to it. If I allow myself excuses/reasons to skip days, then pretty soon I find I'm skipping all days....
*morning - since having children, it's just impossible to consistently do it at any other point. If I don't do it right away, too many things could come up to change the plan, and once I'm off routine, I'm at huge risk for stopping all together. (pre-kids, DH and I used to go to the gym together in the evenings and I LOVED it. I look forward to some day, waaaaaay in the future, doing that again. But it's not going to happen anytime soon and that's okay!) Currently I'm getting up early enough to be finished before my kids wake up.
*Sticking to things I like - I used to equate "exercise" with "running". I spent years trying to make myself run and be a runner. I've never, ever liked it, and anytime I've done it for a consistent amount of time, I end up with some (minor but not comfortable) injury or issue. Once I accepted that I don't enjoy running but there are other ways to exercise, it's been much easier and pleasant to stick with something. I enjoy weight work (primarily though workout videos with free weights), rollerblading (though I don't have a great place to do this right now), walking (fast pace/long distance), and biking. Like OP, I love swimming, but it's not realistic right now either.
*Having a "buddy" - I don't like working out with other people, but it's extremely helpful to me that DH gets up to workout at the same time as me in the morning. We do it separately, but both getting up together is so much easier. Same with times when we used to go to the gym together -- we didn't even use the same parts of the gym usually, but knowing the other person was going makes it so much harder to just decide to skip for no reason.

The two biggest benefits I find from exercise is that my mental health is better, and my nighttime sleep is also better.