I did this back in 2010. I got laid off, and while I was doing okay freelancing, I got an offer to work for GE, but it required me to be in Erie five days a week (I lived in Philly). This is about a 300 mile drive each way, FYI. I took it. I got an apartment and made do.
Couple things:
Plan for the travel. If you come home every weekend, that's fine, but PLAN for it. It's not an inconsequential expense or time commitment.
Accept the outcome. My marriage was going south anyway at the time, but this put the nail in the coffin for me. I LIKED the time in Erie (no daughter notwithstanding) and it helped me come to grips with a relationship that is ending. I hope, for you, that it gives you the time to reinforce the decision you made to move in with your girlfriend, and if you both handle the change like adults, maybe it makes you that much stronger. Hell, after a year of "apart", changing a diaper will be NOTHING!
Be disciplined. If you're doing this for the money, and you end up drinking the money away or buying big screen TVs, you kind of shot yourself in the foot, no? You BOTH need to be on the same page with this. I don't know your girlfriend (and I'm not implying anything here) but if you're banking a couple extra grand every paycheck, and she's bored and goes to Ann Taylor, you're kind of fucked.
Even with all that (which happened to me) I don't regret that decision one bit. I'm very much a "regret that which I DIDN'T do" kind of person, so you have to factor that in for yourself as well.