I swam competitively for about 11 years and made side money teaching it to everyone from triathletes to little asian kids. There are a bunch of things to do, but the number one issue I see with strong athletes who don't have a swimming background is technique. Strong all-around athletes get their butts kicked in the pool by skinny people and little girls because their lack of technique causes them to spend a ton of excess energy moving through the water. Practice the front crawl: Make almost knife hands and learn to insert them into the water smoothly so there's as little splash as possible, then rotate your body and pull back. Consider having someone tape you so you can look at your technique. You also want to set a rhythm with your breathing and make sure to alternate sides: Good rhythms are every 3 or every 5 strokes for someone looking to do long distance, as you'll need to replenish your oxygen throughout the swim.
For non-technique endurance, when you don't have access to a pool, lay on your back, lift your legs six inches off the ground, and perform short, sharp kicks that come just short of touching the ground at a fast pace for as long as you can. If your legs go out during the 500 and you have to use only your upper body, you're gonna have a rough time.
When you do have a pool, you'll want to warm-up and train intervals much the way people would advise you to do with running. By the end of it all, ideally swim 1000yds without puttering out so you can keep a solid pace for the 500 yd swim.
Note: Robert Allen's advice goes without saying. Swimming is ultra-low-impact, so be in the pool every day you can be.