Longtime swimmer, I concur, if you are fighting technique you will tire out. Check out total immersion swimming for some good drills to improve technique. Can also get a pull bouy and hand paddles to work upper body strength in the water, or fins with kickboard for lower body. Mix those in to regular swim laps, mix in other strokes if you can, try to go slow/medium/fast speeds for a lap at a time. For how fast to see improvements, it depends on how much you swim, how fast and far you are going and how fast and far you want to go. Beginners on a swim team go 45 min 2-3x per week and see improvement in 3-6 weeks, with appropriate drills and focus. Hope that helps!