I'm a bit of a plodder, I swim without stopping but I am quite slow. I change it up all the time. Sometimes I aim for 40 lengths, other days 64 (a mile). Somedays my aim is to keep changing my strokes, and do different combinations of front crawl (freestyle), breast stroke and back stroke.
This website has some great drills you can use
http://www.swim4fitness.com/
And four different levels so as you get better, you can make it harder.
I'm swimming tomorrow morning, and I can't wait, I go three times a week at the moment, but am hoping to change gyms soon where there is better pool availability.