i'm not a big fan of swimming, prefer jogging, netball, tennis, climbing to keep active, but when my friends started swimming they aimed to swim 20 miles (not all at once!) in order to raise money for charity. they were, therefore, really motivated and attempted more lengths than they would otherwise have. otherwise attempt say 30 lengths per session at first and then go from there or see how many you can do in 1 hour.
another thought, you could possibly try going to the gym, ost unis have cheapish ones for their students. that way you could try all the different activities and find out what was best for you.