Brown’s plan was not to conduct a sudden raid and then escape to the mountains. Rather, his plan was to use those rifles and pikes he captured at the arsenal, in addition to those he brought along, to arm rebellious slaves with the aim of striking terror to the slaveholders in Virginia. He believed that on the first night of action two to five hundred black adherents would join his line. He ridiculed the militia and regular army that might oppose him. Then he would send agents to nearby plantations, rallying the slaves. He planned to hold Harpers Ferry for a short time, expecting that as many volunteers, white and black, would join him as would form against him. He then would make a rapid movement southward, sending out armed bands along the way. They would free more slaves, obtain food, horses and hostages, and destroy slaveholding morale. Brown planned to follow the Appalachian mountains south into Tennessee and even Alabama, the heart of the South, making forays into the plains on either side.
(http://www.civilwar.org/hallowed-ground-magazine/Fall-09/purged-away-with-blood.html ; http://en.wikipedia.org/wiki/John_Brown's_raid_on_Harpers_Ferry#Brown.27s_Preparation )Through small bands of trained and armed men, “Captain” Brown intended to assist and guide slaves in their escape from the South. The causeway to freedom would be the Appalachian mountain chain, stretching from northern Alabama into Pennsylvania. Night after night, from mountain fortress to mountain fortress, a trained army of liberators — operating in small groups — would escort the escaped slaves northward. Brown’s system would be so safe and so reliable that it would entice thousands of slaves to participate.
Unfortunately for John Brown, almost no slaves heard of his raid before the Virginia state militia surrounded the arsenal. So, he was cut off from supporters and defeated. Yet, what if he succeeded? Specifically, suppose that before the militia arrived, he evacuated the arsenal with as many weapons and supplies as possible for some mountain hideout and prepared to carry out his plan? What would happen?
The slave owners would be terrified, of course - look at Nat Turner's rebellion to see what might happen. The militia would immediately start hunting for Brown. I'm not sure whether he'd be caught; note that most of the people in future West Virginia at least didn't mind the thought of abolition (of course, aiding an armed insurrectionist would be another matter). One way or another, though, this would have great effects a few years down the line, when Lincoln is elected (assuming he still is - do you think a more successful uprising would make the North more sympathetic or more opposed to the South?) What do you think would happen?