How "hard" should I swim? Relaxed vs. strong

So I'm 50, swimming a few years, had a little coaching. Turn 50 yd laps at around 55 seconds, and can sustain that pace for 15 laps or so at a time. Goal = 30 minute mile (35 laps at 51.5, I think). So I understand the strong core and relaxed arms, but I wonder if I'm too relaxed. So, to my question: When I played basketball, we were told that, if we wanted to leap as high as possible, jump at an 8 or 8.5 on an effort scale with 10 as maximum effort, so that we didn't tense up. Is it about the same with freestyle--especially with arm motion? I can sprint a 40 second lap, but even swimming an hour a day, I'm wondering if I'll ever be able to pull off the 30 minute mile? Thanks for any advice on technique or workouts, in addition to my primary question about how "hard" to swim.