I think you'll find it's in there. I don't have the book to hand, but there's a little table somewhere about modifiers due to exhaustion of various sorts, and I think "been doing magic continually for hours" is in it. Similar rating to "been fighting continually for hours" IIRC.
Of course, coming up with interesting results for "magical fumbles" can limit use of magic, too, if you're that way inclined. Mess up a Feat badly enough, the Narrator can deem that the resulting "injury" is you losing your confidence in your ability to do the feat until "healed".
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