In the AD&D Dungeon Master's Guide, Gygax shows how you can convert between a 5-mile hex map and a 30-mile hex map.
The dashed line represents a larger hex that's thirty miles across the middle; I think this is the approach that's most useful for doing a hexcrawl.
The solid line shows a hex that's thirty miles on a side. I think this would only be useful if you were counting distance along intersections, like in Go, instead of from the center of one space to another, like chess.
Here's Darlene's original map for the World of Greyhawk. These hexes are 30 miles across. Gygax is said to have invented and re-mapped this version of Oerth and the Flaeness for publication. His original campaign was mapped in stages spreading out from Greyhawk Castle outward, probably using a smaller scale.
Here's a later TSR map using 20 mile hexes detailing a smaller area of that map (Admundfort is visible in each), which doesn't fit any kind of system (perhaps because hex-crawl campaigns were already being forgotten?). Nevertheless, this is useful for showing the kinds of details that are hidden in 30 mile hexes but revealed by just a little zooming in.
Here's part of the outdoor map from Arneson's First Fantasy Campaign, as published by Judges Guild (who did this cartography). In the upper right is Blackmoor, where the first dungeon adventure took place under Blackmoor Castle. In the lower left is the Temple of the Frog, described in OD&D Supplement I, an adventure that took place some time prior to '74 after the campaign had already been running for a while.
"Campaign Map Notes: In starting my campaign, I reserved a small area out of the center of the Great Kingdom map of the IFW's Castle & Crusade Society (a now extinct Medievals group). The basic campaign area reproduced on a large mapsheet outside this book, was originally drawn from some old Dutch maps. Much of the rationale and scale was based on data found with the Dutch maps. Later, the game moved south and we then used the Outdoor Survival map for this phase of the campaign when the exiles from Blackmoor set up shop after the bad scene at Lake Gloomy." - First Fantasy Campaign