swimming

i can proper myself through the water. so technically i can swim. but what i can't do is float or tread water or do any other stroke other than the crawl. i also can't swim for very long distances. so when i read these training advice columns they assume that you can swim at least 400 meters. as of right now i can't what i need is a workout plan for someone who hasn't tried to swim any extended distance for a very long time (last time was in high school in 1976) so i need some advice on training.