ok im studying Karate right now, but i've looked into Kung Fu and Tai Chi and they have similar blocks vs. certain attacks

recently at lunch this big dude, about twice as tall as me, and twice as strong started messing with me at lunch...it wasnt too serious and it seemed like he wasnt punching with his knuckles so i just ignored him

however, i noticed that his punches were often unpatterned and quick...unlike a lunge punch i couldnt read when it was coming or when his other arm was coming in...
my question is, how the heck am i supposed to defend against something like this in a street fight?

if i dunno when stuff is coming at me, how do i block it...do i just constantly keep moving? please help...also if you have similar experiences, please do tell about them and what you did

thanks for your time

-jeoffrey