Lets say John Brown manages to follow his plan to how he wants it
After managing to gather more volunteer's and kill Robert E Lee and force the marines to surrender. He use's those rifles and pikes he captured at the arsenal, in addition to those he brought along, to arm rebellious slaves striking terror in the slaveholders in Virginia. On the first night of action, 200-500 black slaves join his line. He sends agents to nearby plantations, rallying the slaves. He holds Harpers Ferry for a short time, due to confusion in federal ranks, Allowing many volunteers, white and black, to join him. He then moves rapidly southward, sending out armed bands along the way. They free more slaves, obtain food, horses and hostages, and destroy slaveholders' morale. Brown follows the Appalachian Mountains south into Tennessee and even Alabama, the heart of the South, making forays into the plains on either side.
So what happens next?