20mph is what I try to stay at on the flats.
My 2-year average speed was 12.5 or so. It made me feel slow, so no more bike computers for me!
My last solo century took 6.5 hours, actual miles 105, so 16.15mph.

Oh, fastest recorded downhill for me was 52 mph.

Uphill, I try to stay in the double digits if I can.