on a side note .... (most probably a lot of bollocks as wind etc also plays a huge part, but I'm a sucker for statistics)
if you look at your training routes, divide the elevation by the distance .... the higher the result, the more difficult the route (based on the time ridden)... obviously, a 1000km audax ride even if it's on the flats is a tough task
examples....
LEL .... 1441km long and has 11,128m of climbing .... divide elevation by distance and you get a result of 7.72
PBP .... 615km ... elevation 4290m ... result = 6.9
I have a 42km route that I ride often and that has 300m of climbing ... divide 300 by 42 and the result is 7.17
I also have a route of 135km that has an elevation of 1204 meters (my toughest training route) .... divide the two and you get a result of 8.9
just a guideline so take it with a pinch of salt