The whole medical field even says to walk around on a plane. I use to work the front desk of a physical therapy office & they always said every 15 minutes or so. The circulation in your legs is the thing. Keeps blood from pooling, etc. Plus its good for your muscles.
The other thing for a long flight, after you get on & get settled, take your shoes off. Walk around in your socks except for when going to the bathroom. It helps with circulation since your shoes/sneakers, etc. won't put pressure on the circulation.