Interesting thread.
I know one pilot who got blood clots in his lungs from flying 767's. It can happen to anyone.
I was lucky to fly the DC9 which didn't have recirculation of air and I enjoyed the air for the most part on the old 9. also, the 9's legs at about an hour each allowed for stretching the legs as one did the walk around.
On the 9 there were actually small places one could put their feet on the instrument panel...actually designed that way.
I AM NOT A DOCTOR, but if I were flying long legs I would drink lots of water, walk around and take an aspirin or two to keep the blood moving...AGAIN I AM NOT A DOCTOR.
Please note that Boeing is going to max out their cabin altitude on the new 787 at 6000' instead of the present 8000'.
j