Marin Bolinas Ridge with Specialized Fatboys running 100psi rear and 90 psi front. With rear panniers only 22KM/H average, no tent or sleeping gear and included lots of hills. This was over a trip of 1000kms.
With front and rear panniers, tent and sleeping stuff average speed dropped to 20KM/H over similar distance and territory.
There were a few 6-7 hour days in the saddle during those trips.
Next time I'll get rid of the MTB forks for road touring, ie buy a proper touring bike!