Marine corps infantry m.o.s.?

Well, tomorrow is the day I am going to enlist at 6 p.m. sharp. I am nervous about wanting infantry, although it has been my goal since i did a report back in fourth grade. I'll be 18 in 2 weeks. Anyways, I am 5'9 and about 160 lbs. I'm no shovel face with huge mussels. What will it be like for me in the infantry and could i not get my m.o.s.? I have great stamina and I can push myself harder than most people I know. I hunt and target shoot constantly in good ole' Pennsylvania. (I figured its related to infantry..) What you any of you think i should know about the marine corps/ infantry/ anything else. Please give me any useful information I really wanna know everything i'm getting into. Thank you in advance