I completed Marine OCS a long time ago- 1991-92. Back in the day, the PFT involved situps instead of crunches. Lots of running, calisthenics, etc. They have a number of obstacle courses, rope courses, etc. as well. There were a number of timed runs in boots and with rifle over an obstacle course as well.
In any event, Radix nailed it. I would just add that the running/humping at USMC OCS is really intense. There were a number of people who got hurt/heat stroke/washed out of the program as a result. A lot of people's feet got torn up as other posters noted. A strong upper body is important, but I'd focus on running/endurance if I were you. Good luck.