Being physically fit and being acclimatized are two different things.  And response to high altitude is highly variable.  Probably you will find you are going at a slightly slower pace and breathing more heavily, but you will still get there (and back, which is really the ultimate objective).

Just be mindful of the weather.  Spring can be unpredictable.  There is nothing between Flagstaff and the North Pole except a barbed wire fence....

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