Yes, it may seem a bit strange at first, but in DD (and in OD&D) the duration of a "turn" in the game world is elastic. When you are zoomed in, a turn is short. When you are zoomed out a turn is longer.
The notion of a "turn" is a concept primarily for players; it is simply a period in which they can have their PCs "do something".
During wilderness exploration a turn is a whole game-day in duration. So, at that level of abstraction, the players can have their PCs do one principal thing during a game day. Which is typically to travel from A to B.
If the player is a spell caster, he would usually be allowed to invoke one spell as he travels from A to B--especially if that spell somehow affected his overland travel rate; e.g., haste
, or fly
, or even light
(if travelling by night). Other spells might be reasonable too, at the referee's discretion.
The duration of these spells is still in "turns" (i.e., game-days during wilderness exploration), but whether this means a single casting lasts for the entire duration--or the magic-user is repeatedly casting the spell to make best use of its function over the duration--doesn't really matter. The wilderness exploration game is simply not concerned with that level of detail.
If the magic-user is travelling for many consecutive turns, it might be possible for him to accumulate a number of active spells, casting a new spell from his repertoire each turn.
Should an encounter occur, then the zoom scale will suddenly shift to one minute combat turns
. At this point it's up to the referee to determine what state the magic-user is in at the start of the encounter. I would rule that whatever spells were currently "in effect" prior to the encounter are still "in effect" as the encounter begins, with some sensible proportion of their duration already spent.
Hope that's a helpful explanation
the Elf Medium (MV 12", AC 9, HD 1, hp 1/1, AL N) great cloak, lantern
; spells: color spray; scrolls: sleep, sleep, charm person
Hirelings: Georges; torch