Yup, thats what happened to me. I waited till I had finished the SP and got the 100% before jumping into MP. The high level players just go around on their bulls and snipe you using dead eye. Its really annoying.
Just go private and grind the hell out of gang hide outs to level up. I must have played solomons folly atleast 500 times to get to level 50. My best time was something like 70 seconds and earning over 2k xp with a 30 kill chain bonus.
How long did it take to reach 50 mate?