Questions from a former runner

Hello everyone, I am a 40 yr old male who has been a distance runner most of my life. I have logged countless miles training for marathons all over. Unfortunately I got old :cane:. About a year ago I had minor knee surgery and decided that my running hobby was no longer something I could sustain if I wanted to have knees left when I get to 60. I took up swimming once the knee healed. I am very thin and have never been much of a swimmer. I apparently don't float which makes swimming hard but I keep plugging away. My goal is to stay in shape and maybe get some of those swimmers broad shoulders :). My question is what kind of program should I be following? I am not very good at any stroke besides freestyle and I currently swim for about 30 min a day/6 days a week and do either weights, yoga or elliptical machine for an additional 30 min a day/6 days a week. Given my age, goals, skill level and time constraints, can someone point me to a good set of workouts? Thanks, J