There's a table in the PHB that breaks it all down for youhttp://www.d20srd.org/srd/movement.htm
But basically, 1 mile = 5280 ft ~ 1.6 km
It all depends on movement rates, but if you have any halflings, gnomes, dwarves or anyone of medium encumberance, you're probably looking at 20 ft.
So, 100 miles in 6-7 days on a road
Over trackless plains 8-9 days
Over trackless desert 12-13 days
About half those times if on riding horses, heavier horses would be somewhere in between.
edit: damn, took me too long to type all this out, beaten to everything.
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