Why do your legs feel heavy when you get out of the pool?

Answer 1

When on is in the water ( Which is a denser medium than air ) one is buoyed up .

This is because One's weight is supported by water

When one is out of water and in the air ( A thinner medium as compared to water ) one is led to believe or one feels that he is a heavy weight !!.

Answer 2

when you walk your heart pumps in a lot of blood into your legs as your legs are doing the work. In bodybuilding people call it as pump. The amount of pump that you get is directly proportional to the intensity and duration of your walking.
In short when you walk you carry more blood in your legs and it makes it feel heavy