Strengthening exercises

Hi all, I'm a very late onset swimmer. I took up swimming at the age of 33, after a lifetime of tennis (a lot of tennis). As a result, my right side is very strong, but my left side is really only along for the ride. It wasn't a big deal until I started doing some longer swims about a year ago. I was having a lot of pain in my left shoulder. My esteemed coach, Chris Colburn, saved me by pointing out that my left hand entry was messed up and I was dropping my left elbow. I'm still trying to fix that, and think of almost nothing else (aside from food and sex) while doing my long swims. My left side is still very weak. I tried wearing a paddle on my left hand last night while swimming and I could barely pull it through the water. I really need to strengthen my left side and I was wondering if anyone had any suggestions on exercises. I bought some of those elastic cords but I'm a bit bewildered as to the best way to use them to improve swimming.