Swimming the mile...how do you do it fast?

I was a HS swimmer and just got into masters swimming last fall after 12 years out of the pool (I’m 30). My primary goals are to improve health, lose some weight and have some fun. However, I'm longing to go faster. Right now I struggle to hold 1:20s for more than 500yds and feel wiped out. I'd like to really improve on my mile distance, as that's about the distance of the Triathlons I like to do (oly & 1/2). Does anyone know of any good resources that can teach me what to do to swim faster over a long distance? In other words, how much to I need to swim, what kind of sets etc. should I focus on to reach my goal of holding 1:10 for 2000 yards and have enough left in me to be ready to hop on my bike and ride for 56 miles and then run 13.1? Right now I swim 3-4 times a week with 3-4000 yards per workout. I swim the workouts that the coach puts up, and don’t swim on my own. The workouts I think are designed toward the “overall” swimmer, not to distance freestyle. I’ve been seeing steady improvement, but can I do something else to “help” it along?