TL:DR - under optimal conditions, an unarmored PC in Classic can walk 1 Hex per Watch. One Hex is six miles across; one Watch is two hours.
Get outside. See what three and six miles looks like first-hand. Use those drones to get aerial shots if you can so you can get a good idea of what a bird’s eye view of that is in the world. Kingdoms are not as big as we’d like them to be, which means that the tales of heroes and rogues like Conan, Fafhrd & The Gray Mouser, Locke Lamora, etc. make a lot more sense in a far more localized milieu where you could escape the undesirable consequences of one’s actions just by skipping town and heading to the next one over.
It worked in the Old West. It will work for you. This also justifies the presence of high level characters in what we wrongly presumed was too dense an area. No, it’s not, and it’s entirely believable when you consider how closely the Sheriff tracks with the Lord.
When Classic adds hex rules, it’s going to be 6 miles. It just makes too much sense:
it takes two hours walk on a flat road - I do an 8.5 km walk most mornings, it takes me 90 minutes. 120 minutes for a 9.7 km is leisurely when walking on a paved path.
you can barely make out things 6 miles away, and make out things 3 miles away. This assumes a straight unobstructed line of vision, in reality it will be much less because of trees etc, unless you’ve climbed a hill or something. So from the centre of a hex, you can kinda see what’s in the next hex. In the picture at the beginning of this post, the building on the hill is 3.5 mile away.
it maths nicely: 1 hour gets you 3 miles. 4 Watches (2 hours each) gets you 24 miles (which is on the high end, but OK). This is the best you can do: it will scale down as the terrain gets worse.
Movement in D&D tends to involve 3s. In AD&D, an unarmored human moves 12”, so we can say 12” = 3 miles / hour in this hex system. In 5e, it’s 30 feet / turn, so we can just say “divide by 10 to get mph”. In Classic base movement is 30, so divide by 10 again to get miles per hour. Divide by 30 to get Hexes per Watch.
Teddy Roosevelt wanted his officers to walk 50 miles in 20 hours over three days. We’re doing 48 miles in 16 hours, a bit more aggressive but our PCs are not sitting at a desk all day.
As per the quoted text above: you’re basically less than a day away from new adventures and easily escaping your sins.