The travel pace rules say they are meant to represent a group of travellers of differing capabilities. In effect, they are rules of averaging, in that they:
assume that a group of travelers adopts a pace that, over time, is unaffected by the individual members' walking speeds. . . . [D]uring an overland journey, the difference [i.e., in their speeds] vanishes as travelers pause to catch their breath, the faster ones wait for the slower ones, and one traveler's quickness is matched by another traveler's endurance.
DMG p. 242. The overall effect is to treat all such travelers as if they have a walking speed of 30 feet and calculate pace from there. For simplicity, let's focus on the hourly pace -- a "normal" (i.e., not fast or slow) pace given as 3 miles per hour. See PHB p. 181-182.
The rules also contemplate a mounted character traveling at double pace for an hour, because "many animals move much faster than humanoids." PHB p. 181.
The DMG also includes "special travel pace" rules to reflect a creature traveling via "magic, an engine, or a natural force (such as wind or a water current)," which wouldn't "tire the way a creature does" and might avoid "the types of obstructions found on land." See DMG p. 242-243. In that case, formulae for special paces are provided. The special formula for hourly pace is walking speed divided by 10.
These rules appear to leave a gap, however, regarding creatures traveling alone -- unhindered by slower companions -- but without the benefit of either a mount or a means of tireless locomotion. All else being equal, a rogue with Cunning Action can move significantly faster than other creatures by dashing. Nevertheless, that fast movement is still movement on foot; it is not "magic, an engine, or a natural force." And from a "what does the character sheet say" perspective, it doesn't change her walking speed.
If a rogue with Cunning Action (which can be used to dash) and a walking speed of 30 feet travels overland, what is her pace? Is it:
- 3 miles per hour, because the normal travel pace rules apply and Cunning Action does not affect them?
- 4 miles per hour, because Cunning Action represents a "fast" pace under the normal travel pace rules?
- 6 miles an hour, because the special travel pace rules apply and with Cunning Action she can move twice as fast as a character with a walking speed of 30 normally could?
- 6 miles an hour as above, but only for an hour because she is functioning like a galloping mount?
- 9 miles an hour, because the special travel pace rules apply, with Cunning Action she can move twice as fast as a character with a walking speed of 30 normally could, and she can make fullest use of her mobility because she is alone?
- 9 miles an hour as above, but only for an hour because she is functioning like a galloping mount?
- Something else?
Does the answer change if the rogue is affected by longstrider?
A rules-as-written answer gets bonus points, but any persuasively-reasoned answer is acceptable.