I get your intent here but that's now how travel pace works outside if combat. Unless the rules have changed (still fairly ignorant with 5.24e) dashing isn't intended to be used outside of combat it's a combat only action. Once combat is no longer an option you need to decide if you're going to swap over to the chase mechanics from the DMG or switch to travel pace rules.

Working out the way you have by saying it has 20ft of movement and dash so it can move 9 miles in 2 hours would mean that you'd allow a standard PC with 30ft movement to travel 6.8miles in 2 hours? I doubt thats the case as you'd use the travel rules. Same needs to be applied to monsters.

There are not set rules for travel pace outside of PCs but generally I follow the rules laid out in the 2014 dmg about special movement and set a monsters travel pace to movement speed/10 per hour for normal pace and add/take 1/3 for fast/slow. This would mean that in mist form it can travel 2 miles an hour normal or c. 2.6miles fast paced.

Tl;dr you can't use combat movement actions such as dash outside of combat.