In the DMG pg 242 daily travel for humanoids would be 24 miles per day "normal" pace, 30 miles a day fast pace.
If you assume humanoids average speed is 30ft per round and an adult dragons flying speed is listed at 80 ft per round. You could calculate a ratio for "normal" pace by 80ft/30ft * 24 miles/day = 64 miles/day.
If the dragon does not need sleep perhaps add half again more distance, assuming a humanoid travels 16 hrs in a day. So dragon "normal" pace, no sleep 64 miles/day x 1.5 = 96 miles/day. If you assume humanoids only travel 12hrs per day then you would double the number for a sleepless dragon, so 64 m/day x 2 = 128 miles/day.
To get MPH you just divide the distance by number of hour not sleeping. So since, 24 hrs flying with no sleep gets you 128 miles. Then 128 miles/ 24 hrs = 5.3 miles per hour.
If you throw in the DASH action for every round then you can double the above numbers calculated. So max miles per day no sleep 256 miles and speed = 10.7 MPH.
Actually there were a lot of variables, no description on what was considered when calculating the humanoid travel times. Was it flat straight travel or in the mountains, certainly flying would be straighter. Loss of time for up and down hills around obstacles .vs. flying as the 'crow flies' straight line flat travel.
How do head wind or tail winds effect them? Some arguments indicate that dragons are too heavy for the wind size to fly unassisted by magic. So the do the normal rest, eating and drinking breaks apply that would for normal animals.
A top speed is probably different than a speed that can be maintained for hours on end. I just tried to give you a starting point to work with for long term travel times. Lots of room to throw in other assumptions and change the values.
As Dan B noted, if you go only on the rated speed listed for Dragons 80ft per round (assuming a round includes both movement and action then if you are only moving and effectively are using a "Dash" action, you might double that for short distances at least, so 160 ft per round. 600 rounds in an hour so 160 x 600 = 96000 ft per hour = 96000 / 5280 ft per mile = 18 mile per hour.
Here is a link to bird speeds, but even for them, the range vary from 15 mph seagull cruise speeds to 200 mph for a falcon diving on prey. If dragons use magical assistance for flight normal bird speeds do not have to apply.
You might argue that dragons could be pseudo-levitating behemoths that only move a few miles per hour. That a small sail boat can move much more quickly than a huge cargo ship. Certainly turning for a dragon would take much longer than for a sparrow.
Dragons at the least could have developed slower moving modes like maybe hover, so they can land safely and pluck prey from the ground without crashing in to trees or over flying prey or they would only be able to hunt in flat open areas.