this is what I have been doing for the last 2 weeks.
I know I'm an iron duck, so I have been swimming for 2 weeks now, 4 times a week. No more running. Still doing weights and everything else. Before, I couldn't even swimming for 25 yards, now i can swim 100 yards straight, but slow. I notice swimming has helped me a lot with my breathing. Therefore, I decided to run on a treadmill again today. Good news, I went from 13:30+ to 12:20. Bad news, my knees and back began to hurt (never felt hurt before). So this is what I decide to do starting next week: instead of swimming 4 times a week, will swim 2 days, and run 2 days. Let see how it will turn out.
If you want to improve your run time, try to mix it with swimming, as it worked for me.