When you first enter the Army you attend what's calle Initial Entry Training (IET) it's split into two parts BCT and AIT.
Your Basic Combat Training (BCT/"basic training) local depends on what MOS you picked. If you have a combat arms or MP MOS you will go to what's called One Station Unit Training. You stay at the same location for both basic and AIT. You have the same Drill Sergeants for both and they will have your MOS. THey have a pretty good idea of what it takes to be sucessfull in your choosen filed. The reason that combat arms troops seem to do more PT and the instructors are harder on them is BECUASE THEY HAVE TO BE. The military is not a game. They are trying to provide you wil a set of skills that may one day save your life or ther other lives of other soldiers in your unit. I don't want to sound harsh, but if you are not sure you are ready to commit to it, then DON'T.
Remember the Army and Marines are ground combat forces. Their primary duty is to seek out the enemy, find him, and KILL him him. It's that simple. If you have a fear of physical activity, LONG hours, poor food and living conditions, then you may want to start looking at another profession.
Don't sign up just becuase you think its cool, or its what your frineds are doing make your own decession, becuase it one you will have to deal with. unlike most other jobs, this may actually get you KILLED.
I'm sorry if I seem blunt, but its just the way it has to be. There are entirely to may people joining the military that think its some kind of summercamp, or a quick way to earn some money for school. They take a check every month, and when crunch time comes they can't cut it.
Please, please, please, think long and hard about your future.