probably too late to respond but...
with garen, it's very obvious when he's going to all-in you.
he will always start with his Q to silence you.
my advice: run. if he doesn't get to you in time, his Q will go on cooldown and he'll do a full 180, trying to get away from you. that's when you go in.
you draw out his Q (don't get too close, it grants him temporary speed), and punish him hard.
garen's R is almost always used to execute and will usually only be used if he knows for sure that he can kill you. this is why you do NOT let him all-in you unless you have some sort of level/champion/item advantage and will 100% win.
this leaves him with his shield and spin. his spin does the most damage since it's usually maxed first, so either shield or get out of range.