Need some beginner advice

Hi, I thought I'd register to ask some questions. My background is that for the past 6-7 years I've been bicycling like crazy, often times 10-15 hours per week. I'm have the stereotypical runner/cyclist build, big legs, and I've always felt like my upper body was not nearly strong enough to balance out my lower body. I'm 29. Also from about 15-19 I worked as a lifeguard and swim instructor. However I've never swam competitively. My strokes are pretty good as a result of getting through the WSI program and teaching, but I have basically no knowledge of how to actually swim for fitness. (Except for butterfly, I can swim it, but I probably can't even swim 25 yards of the Butterfly before my stroke breaks down and I'll be heaving when I get to the end of the pool.) Given I've gotten sick of bicycling through New England winters this year I finally have a pool convenient and I've gotten pretty hooked on swimming laps. However given I have next to no knowledge of how to train I seem to be getting myself in situations where I can overtrain myself very easily. Right now I'm generally swimming 30-45 minutes at a time, 2-3 times per week. I'm not swimming in a full size pool so it's tough for me to judge distance but I'd say it's about 12 yards as opposed to 25. When I swim I'll generally swim about 20 lengths of this pool before switching my stroke. I'm not really taking any breaks but about every 40 lengths I'll swim 10 lengths of elementary backstroke or sidestroke to try and rest. What seems to happen though is if I combine in *any* other activity such as running, weight lifting, even going snowboarding, I very quickly show overtraining signs. (Fatigue, trouble sleeping, appetite/stomach issues, etc..) All the swim workouts I've seen on the web seem to focus on distances and intervals, I'm kind of lost as to what is an appropriate time for someone like me to be swimming in terms of hours & minutes as opposed to lengths. Should I be resting after a certain # of laps? I'm going to try wearing my heart rate monitor tomorrow to get an idea of how hard I'm working at each stroke, compared to my normal cycling workouts it seems like I have a harder time gauging my intensity level with swimming. Any advice would be appreciated. I don't really think I have a convenient way to go join a club and swim with other people to pick up some knowledge, but I really need to given I'm the kind of person that easily pushes workouts too hard. Thanks!