How to get the best results swimming?

My buddy and i are enlisting in the navy, and its been a while since we were in football, so weve decided to start training by swimming. I always heard swimming has way better results than running, and ive always done alot of running so i cant wait to see how swimming does for me. However how exactly should you swim to get the best results? Should i swim fast for short distances (in comparison to sprints) or should i swim back and forth without stopping at a good pace, like long distance? My goal is to drop the gut ive gotten from all the partying weve been doing since we got out of school, and to get my legs stronger and more flexible quickly so i can get back into squats. Also how do i get a back like michael phelps lol!? Im sure some back muscle would help with pull ups, how do i start working on my back? thanks