I am pretty sure the author would pick "i have been training hard in secret for years since the day i met you" route.Only thing I ask for is, tell that girl you went to another world.
The art is nice, the action is engaging, don't ruin it with a pointless secret.
It's fine if nobody else knows, but she's literally looking at the guy unleashing his power, surely there is no way the author is going to make up a retarded excuse.