Which civil airport has the longest runways in total?
Best I can find, within the US, is KDFW with 80,403' of total length. Second seems to be KDEN with 76,000'. Glanced at KORD, and with mental math it comes up to about 71,000'. Most other major airports are well behind all of these, mainly because they don't have as many long runways as DFW and DEN.

Somebody else can consider Russian, Chinese, and Middle Eastern airports -- I don't have the chart sets handy for those.  May be worth converting this to a community wiki answer if any of those places give interesting results.