it's slow time of year, so i thought i'd throw up some discussion material
based on ability of komen/wang, it seemed that doubling distance for them ( from 1500 up ) seemed to indicate a reduction in power of 3^0.5 or 1.732
from that, assuming those guys are pretty much the absolute limit of human machines ( runners will obviously get quicker, but their ratio of performances probably won't - & actually even dedicated college guys can get those ratios, they woud just lack speed to run 7'20, etc )
1500 -> 3k is ~ 2.0982
3000 -> 5k is ~ 1.7267
5000 -> 10k is ~ 2.0982
10k -> 1/2M is ~ 2.2216
1/2M -> M is ~ 2.0982
obviously road courses vary, but this woud be for a road as flat & as windless as an enclosed track
remember, those are endurances based on komen/wang, both currently 3k wr holders ( but that endurance is achievable by even college guys )