Would be interested to know how you use the AD&D Monster books and DMG in 5e - I try to steal from 1e DMG random tables and often borrow 1e Monsters/Treasure (the latter especially tending to be a little more flavorful than 5e counterparts) but it's not something I have the facility to convert...
This is why I like having a straightforward open procedure for this sort of stuff, that the players have meta-knowledge of. If you leave your horses unguarded outside of the dungeon while you're delving, I'm going to roll daily or whatever to see if anything happens, probably with stated odds...
My quick and dirty approach for hex-crawls (where this most often comes up for me) is 1.5x "standard" PC walking speed, rounded to nearest hex in open terrain (roads, plains). So with 12 mile hexes, a party going 2 hexes a day on open terrain goes 3 if mounted, etc.
I think my ideal 6E would be something in between 2014 5e and AD&D. The somewhat streamlined core systems and middle-ground complexity "character builds" of 5e (by middle ground, I mean between TSR era and my first and second hand understanding of 3e/4e/The Pathfinders), combined with the larger...
It was Anthony Huso's blog (and modules) that got me interested in AD&D - I had sort of ignored 1e, coming from a background of 5e and some OSR B/X variants, envisioning it as kind of clunky and arcane. Huso seems to play a rather (but not entirely) RAW version, and makes a lot of the arcana...