Brendan at Necropraxis writes some very reasonable things about hexes and wilderness exploration. His is a fine solution.
People get too hung up on size of hexes and miles per day of travel.
The Underworld & Wilderness Adventures provides a beautifully abstract system, particularly if you ignore the recommendation to "assume the greatest distance across a hex is about 5 miles," and leave the size of hexes abstract.
In OD&D, each means of travel affords a certain number "moves" per day. A man on foot gets 3 moves, a draft horse gets 5 moves, and a light horse gets 10 moves.
Each terrain type has a "cost". Following a road or track through most terrain costs one move. Bushwhacking through the woods costs two moves. Crossing mountains or swamps costs three moves.
So, a man on foot could "spend" his three moves to cross three hexes of good road in a day. He could instead spend his three moves to cross one hex of swamp.
The OD&D system isn't necessarily better than the system proposed by Brendan (and Brendan writes about more in that post than just travel rates). One hex per day regardless of terrain would work for me. But it would take a lot of convincing to get me back to converting dungeon movement rates to miles and dividing by the sizes of hexes and accounting for terrain types and fatigue so on.
Keep it abstract. If the model is good enough for players to make meaningful choices, it's realistic enough.