I don't think spending many hours in a seat on an airliner is any different from sitting for several hours on a bus or, for that matter, in front of the computer for hours and hours (which I and many others do).

Moving about in an airliner, especially one flying over areas where severe turbulence is common (through jet streams, for example) is probably a greater risk factor than DVT, since you could end up breaking your neck against the cabin ceiling in the event of a turbulence incident. When I fly, I remain in my seat throughout, with the safety belt on, and I recommend everyone else to do the same.