I would consider the following assumptions to be true:
#1. The army, if it is to remain whole, can only move as fast as its slowest unit types. So, if there are heavy infantry on foot, per our wilderness movement rules (Player's Manual, p. 257), heavily armoured types can achieve about 12 miles per day.
#2. Also on p. 257, I would take into account the terrain type being crossed.
The information in that section of the book (starting on p. 256) should contain everything you need, including for the mobilization of larger forces.
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