I agree with Joe about the escape route taken by Huck and Jim.
I also recall that another reason for going to Cairo is that Jim would stand
chance of escape in a city large enough to have a chapter of an abolitonist
society or a
branch of the underground railroad. Joel Chandler Harris writes of
(Patrollers) in Georgia, which I'm sure existed in Missouri and Illinois as
patrolled, especially the border, looking for runaway slaves, seeking the
bounty as Joe
said or re-sale. Jim could not know what type of people he might encounter
or whom he
could trust simply crossing and walking through unknown countryside without
direction to take or a place to go; he might add to his chances of being
Aggitation that would result in the Fugitive Slave Act, intended to increase
pressure on non-slave states to return runaways, is also already in the air.
Of course, Jim misses Cairo (Kay-row) and, as though it were Cairo (Ky-row),
himself in danger of "being sold into Egypt" when they journey into the Deep
("sold down the river" is the phrase that often appears).