Lots of factors - but I agree with Valygrl - 60 +/- 40.

I generally use a 10 mph rule of thumb.
That included banana, photo, and water breaks.
Speaking of water - I think it is more relaxing to get off the bike for a water break.
Maybe pull off under a tree - even if just for a couple of minutes.
So that makes touring different from recreational riding.

On summer days with 14 to 16 hourse of sunlight, it is not hard to do 80 miles.
It depends on when you get started and how late you are willing to go.
I have a fair complexion so I skip the middle part of the day - cycle early and late.
If I stay in a motel or hostel, I usually get a much later start.
If there are lots of towns and stores, I usually take longer and more frequent breaks.
In Nevada, where it may be 80 miles between water -
One has a tendency to be more focused.

Then there's the wind.
100 miles with a killer tailwind is often easier than 60 miles with a headwind.
Stiff sidewinds can be tiring, too.
I guess the key is flexibility.
If you have a rigid itinerary, you will find yourself exhausted one day
And getting to your destination far too early the next.

My advice (+ 5 cents)
Ride early.
Stop often.
Plan, then discard.