You need to be fit to join the Marines. Unless you are a regular for the local semi-pro football team you will need to prepare your fitness. You will not be helped with your fizz in the RM, you will be expected to be at the required std from the start.
Look up BFT, Beep Test and CFT. When I was in BFT was a squadded run / fast walk (pace set by the PTI) in 11mins followed immediately by an individual run of 10.30 (Marines and Paras need to be able to breeze this test). The Beep Test is a common test (google it), different regiments/corps have different pass standards. CFT was 10 miles in 1hr50 with bergen and rifle (again in a squad paced by the PTI) followed immediately with a fireman's carry and loading a weight in the back of a Bedford.
Check out the educational standards reqd to be an RM and the tests you will need to do before (and after) you join.
Don't be put off, be encouraged, and good luck.