They actually had pre-planned the uprising by word of mouth significantly before the actual strike. The raid on Harpers Ferry was to gain the weapons they would need. The issue is, since Brown’s core force never arrived, the slaves (correctly) assumed that the rebellion was snuffed out, and so did not rise up at all.

You may be right about the rebellion still being doomed, just wanted to emphasize that there was a plan that was not completely insane and unreasonable!