Physical fitness matters a lot but mental fortitude and a desire to reach your goal will get you threw. i made it to the 2nd week of sas selection till i quit i just did not want it as much as i thought i did. All the pre selection tests are quite easy if your fit but once you get to the selection course or in your case your "buds" class its a different story.

Little food and sleep, the mind games they play with you the high potential for injures the staff can drop you at anytime just because they don't like you and the fact that the odds are against you. Id say good luck but passing the course is not a matter of luck you either want it or you don't.