How do I achieve the 1k in 15 min? How to prepare for 4k OW?

First, there was a line in medical aper about exercise progtrams taht said "SEdentary men in their earl;y forties who begin a regular exsercise program show measurable cardiac changes in as little as 6 weeks." I've mentioned this to my boss a couple of times. She's a seasoned anesthesiologist who always makes the point "They mean good changes." Keep it in mind - you can change your muscles, your nervous system, tolerance for pain, and your energetic metabolism easily in 5 weeks. You're rolling the dice if you do all-out 4 K after doing that. I can't say the best way, but a few years ago, I trained for a 3.6 k race from near-sedentary. The 100 yard training could probably use a different approach. Here's what I did. Started with an easy 600 yard mixed stroke swim in the pool (Free, back, ***, kicking, no fly.). Worked on increasing distance 3 X per week until I was up to 4000 yards (about 6 weeks), then switched to one weekly distance, one weekly paced mile and one weekly interval set. I spent another 6 weeks or so doing that - longer distances, faster miles, faster intervals. Last 3 weeks, I shortened and toughened the distances and upped the mile to 1.5 mile timed swims I could have done better with 1) more weeks 2) some stroke instruction. Typically, my "distance" workout took me a bit longer than your 1.5 hours and the pacing and intervals took less time than that. I found I made faster progress when I gave up doing "days-in-a-row" workouts. YMMV. Good luck.

First, there was a line in medical aper about exercise progtrams taht said "SEdentary men in their earl;y forties who begin a regular exsercise program show measurable cardiac changes in as little as 6 weeks." I've mentioned this to my boss a couple of times. She's a seasoned anesthesiologist who always makes the point "They mean good changes." Keep it in mind - you can change your muscles, your nervous system, tolerance for pain, and your energetic metabolism easily in 5 weeks. You're rolling the dice if you do all-out 4 K after doing that. I can't say the best way, but a few years ago, I trained for a 3.6 k race from near-sedentary. The 100 yard training could probably use a different approach. Here's what I did. Started with an easy 600 yard mixed stroke swim in the pool (Free, back, ***, kicking, no fly.). Worked on increasing distance 3 X per week until I was up to 4000 yards (about 6 weeks), then switched to one weekly distance, one weekly paced mile and one weekly interval set. I spent another 6 weeks or so doing that - longer distances, faster miles, faster intervals. Last 3 weeks, I shortened and toughened the distances and upped the mile to 1.5 mile timed swims I could have done better with 1) more weeks 2) some stroke instruction. Typically, my "distance" workout took me a bit longer than your 1.5 hours and the pacing and intervals took less time than that. I found I made faster progress when I gave up doing "days-in-a-row" workouts. YMMV. Good luck.