I'd be really annoyed I have to take classes again, and I'd be kind of f-ed in some of his classes. Oh, and I'd miss my wife and kids, but hopefully it's temporary, and I leave the MC a whole bunch of notes on what to do that assumption.
If I end up stuck there, I live very conservatively because at almost-50 I feel a lot more mortal than an almost-17 year old should, tell the MC's best friend not to take any dumb risks, and start engaging in some major spoiler powers. Although some of the biggest secrets I (as author) know, nobody in that would would believe coming from someone MC's age, a couple of them I'd know how to prove [faster than the MC ends up finding them], at least if I have the MC's ability to learn magic.
I'll ignore any question of the ethics of dating a character whose backstories I wrote, nor of being mentally old enough to be their father. Having said that, I'm not sure author knowledge would help much with MC's love life at least without those chapters already being written. There's a lot of self-insert to the MC, but he's a lot more of a classic introvert, while I was at that age (and to some extent still am) more of a weirdo.
Also, depends a lot on when I go back into the story. If it's early enough, a lot of things go differently; with author knowledge, Part I goes really differently.