beginner workout swimmer

Hello, I'm a 33yr old female with two children, 3 and 8 months. Before my second pregnancy I was trail running and I worked out during my pregnancy. However, my husband is building us a house and I basically have NO TIME to workout. I'm not completely out of shape and not overweight but..... I swam in the Y pool the other day for the first time and was WIPED OUT after the first two laps. I plugged away for another 15 minutes but had to keep stopping after each or every other lap I was so out of breathe. What am I doing wrong? How do you "workout" in a pool? I would like to do a triathalon in the early fall. I like to swim because its easier on your body and my knees are starting to ache after I run. Also, there is a 60 year old woman who swims at the Y and she looks GREAT, very good advertising for swimming, this woman. I would appreciate any and all comments on how to workout while swimming? Thanks