I am thrilled to meet other swimmers. I'm a former competitive swimmer (way long time ago, I remember nylon baggy suits, and when goggles first became available!) So I've been swimming a long time. I think the Total Immersion swimming book is just great and well worth the investment. You can also look on Masters Swimming sites for tips, workouts, etc - you don't have to be interested in competition to get good info there.
I think the interval training sets Shades of Gray submitted are a great idea. Just doing laps can get boring and also it's easy to become comfortable and in a rut, and stop progressing in your strength and endurance. Interval training also raises and lowers your heart rate, which is a good thing.
http://www.swimmingworldmagazine.com/swim-cgi/ Also, try this link to Swimming World Magazine for workouts - you can put in the various parameters according to your stamina and skill level and it will give you workouts submitted by readers, etc. I've tried some and it's been helpful in giving me new ideas, even if I don't follow the whole workout or do things just as they say. Maybe it's not right for you now, I don't know, but if you keep swimming it may be helpful. (I remember subscribing to this magazine as a young person and in the first issue I received they were discussing the 1968 Olympics, with Debbie Meyer on the cover. I saved it for years! Things have certainly changed!)
Good luck. Swimming is such good exercise, so nice for mental relaxation, and really rewards persistance pretty quickly, I think.