How quickly you fall out of shape

Yesterday I didn't have track practice because the boys basketball team was playing in the regional semifinals (they lost), so I took the time to do some extra training. I went to the gym and ran three miles on the treadmill and then went for a swim. This is the first time I have swam since my eye surgery, which was four weeks ago. At that time I was able to swim 1000 yards continuously and probably could do more. Yesterday my plan was to swim about 800 yards. I quickly learned, however, that I had fallen out of swimming shape in that four weeks. I was able to do 400 yards and struggled throughout. I might go back to the gym today after practice to do a little more swimming. I think it helps my running a lot and I need to get on it if I'm going to do the triathlon this summer.