Swimming again, its like riding a bike....?

...or so I'd like to think! I am a 25 year old female who swam competitively for 2 years when I was younger (10-12) and I played many other sports as well. My coach at the time told me to pick a sport, and let's just say I didn't choose swimming. I chose softball, which ultimately led me to college scholarships and paid my way through college. I graduated several years ago and play the occasional rec ball. I DO NOT LIKE going to the gym and running since the 12 years of catching has done a number on my knees, so you can imagine how great it feels to get a good workout in without pain afterwards. I just joined the YMCA this week and we have 2 pools, a 25 which is out of commission for the winter, and what I think is a 15 with a bubble over it. No lane ropes, no lines on the bottom. I am uber excited about being in the pool as I've always loved the water! I was a great swimmer when I was younger and I still have that "itch" to compete. I know that I am not in good shape since I've been sucking some wind, but with some training and hard work, I would consider it again!! So, with 15 years off from competitive swimming, where do I begin? I have been doing laps in the 15 the past 3 days and I feel invigorated, not to mention reading everyone else posts!! Great motivation, thank you all!! My real question is how can I do great workouts in a small pool while being out of swim shape? HELP!