I'm not sure it's helpful to just use a generic term like "hills".

A "hill" could be gentle incline, it could be a short sharp pull, it could be a monstrous great thing. Obviously a hill that is long and steep will have a much bigger difference than a hill that is short and gentle. It also depends on how well an individual rider copes with hills, and the overall elevation profile of a particular ride (a return journey will, by definition, end up at the same elevation it started while a one-way trip could conceivably be downhill all the way).

There's a road near me that's a long incline but a gentle gradient. As a rule of thumb I reckon on maintaining 15-16mph going one way and 19-20 going the other way. There's another road fairly near me where if I reach the top without dropping below 5-6mph I'm pleased about it, and coming down I routinely hit 30-35mph without even pedalling.

On my bike on the flats I aim to cruise around 15-20mph and pass most other cyclists at that speed. Every once in a while I come across a guy on a road bike dressed in Lycra who smokes me like a kipper. On occasion I keep pace with a roadie in Lycra, although whether that's because I'm pushing and they aren't or I'm as fit as they are remains unknown.

Of course for good measure you need to throw in the tendency of some folks to exaggerate their achievements online.