How to improve endurance/fitness

I started swimming in mid-July. I took 5 lessons (which helped a great deal) and have been to the pool about 3 times per week since I started. Basically patterning my workout after the general advice my teacher gave me, I currently start by doing 4 lenghts (in a 25 yard pool) of freestyle, 2-4 lengths of kicking and then I pull for 6 laps (because its fun). Then I do 5 50s with a minute between each and then I do either 3 50s or 2 75s of backstroke then I go back to working on stroke by swimming another 10 laps or so of freestyle and/or pulling. When I first started, my 45 minute workout would yield about 16 laps with all of the breaks I required. Now I do about 40, but I appear to be stuck there because I just can't get myself to do anything greater than 50 yards at a time - I did a 100 of pulling the other day - seems that if I'm not using my feet, its a lot easier to get back and forth. When I do my 50s, I feel like I'm going to pass out by the time I'm reaching the end. Perhaps I should just push on and turn them into 75s - but does anyone have any advice as to how I can build a little more endurance? Is it just a matter of pushing myself a bit harder to stretch out those 50s into 75s and then 100s or is there a building block approach with a different workout setup I should try? My goal is general fitness and weight control (can't run because of bad knees and I finally started swimming as an alternative) I swim 3 days per week and lift weights pretty heavily 3 days per week. Thanks.