Modest fitness goals

Hi all, I'm trying to get fit after a 2.5 years spell of sitting on my butt and eating ice cream and chocolate in-between regular meals. I vaguely remember feeling much fitter when I swam and rode my bike regularly, so I thought I'd get back into swimming. I went to the pool yesterday. The 25m pool was booked out for kiddie lessons so I had to go and gently lower myself into the 50m pool instead to get my much awaited workout. The good thing is that I remember how to swim. The bad thing is that I swam all of six laps in the 30 minutes allotted for my swim, and couldn't complete a single 50m length once - I had to stop in the middle of the lane and cling to the wall to get my breath. However despairing the first swim has proven, I'd like to continue on the path to fitness. I know this may seem laughable for the folks on here who are much fitter, but what sort of "workouts" could you suggest for the likes of me? I know about the zero to 1 mile plan but I'll have to work up to that for some time I think as I can't as yet complete a single lap, and the workout starts with 100 yards straight! Any help, advice, and jokes at my expense are much appreciated.