The nice thing about Wolfpack being in the game is that you don't even have to play with anyone else if you don't want to in order to reach level 55. I don't know how long it would take, but in AC3 compared to the other MP modes, I generally earned more exp playing that than the other MP modes.
I did the AC3 achievements under my account back when the game came out last year. But I have been replaying all of the games under my wife's account recently and just got to AC3 last week. Of course there aren't a whole lot of people playing the MP portion of the game anymore. But still I was able to earn all of the MP achievements by level 15 (except the level 20 one of course). And the few matches that I got into after that were filled with idiots or the servers disconnected me from the host, losing all progress.
I decided to just go into invite-only wolfpack and get the rest of the levels there. It only took me 3 games to go from 15 - 20. Since you're the host you shouldn't be able to get disconnected (but then Ubisoft might find some way of having that happen). It worked out really well.
So just a tip for AC4, once you get the other achievements, it might save you some headaches if you aren't fond of playing with others online in this game by playing the Wolfpack mode solo to get the rest of the way there.