There's some interesting stuff being said here, and I'd like to chime in with a bit of a different perspective.
I started OCS with Class 13-18 and rolled to H Class for the IST. I failed push-ups, because my form was trash and I got sharked pretty hard (on the retake Thursday morning I actually failed push-ups AND curl-ups.) I was, of course, not the only person to fail. I and several other people rolled to H for the IST. H Class was an interesting experience and it's not something I regret. I met amazing people there, who were all there for different reasons. Many failed push-ups, some curl-ups, and a fair few failed the run. By a lot, some of them. No one who failed the run was ever kicked out. They were rolled to H Class and given second.. and third.. chances. When they failed the third time, after six weeks in H Class, that is when they were attrited. Someone from H Class who failed the run with a run time of over 18 minutes rolled up to 14-18 with me after the IST retake.
Despite push-ups being my weakness, I was one of the slowest people in Class 14-18. Crazy, seeing as how my 1.5 mile time was/is 13:15, which is between an Excellent Low and Excellent Medium for my age group, but it's true. I WAS that person Aulani is talking about, who fell behind the group on the 3 mile road run. I was the person CAPT Nicholson, my class officer, or my class chief ended up running beside and cheering on. I was the slow person at PT not getting done with sets as quickly as everyone else. I wasn't alone, of course. There were a few other people, but eventually those people rolled or were kicked out (the two who were kicked out were kicked out for other reasons, too, unrelated to PT.) There was one major difference between myself and those who were rolled or kicked out: I tried. I never went to sick call on run days, like the ones who were kicked out or rolled, because I thought I was clever and could just skip PT whenever I wanted because I didn't want to try. I NEVER fell out of a run, I only ever fell behind. I never stopped. I never quit. I never said "I can't." I never went to sick call or SMART instead of regimental PT. I volunteered to do demerit work offs with my classmates, even though I never got one. I accepted advice, criticism, and encouragement from my class team. I put my heart and soul into every RPT session, every morning's PT, and in every physical evolution at OCS. I never let staff doubt that I WANTED to be there and that I WANTED to get better. And I did.
I and those other slow people, in the beginning, were all counseled several times. Meaning we were called into an office by someone in our class team and formally counseled about how we were failing to lead from the front at PT. I am the only one out of those people who graduated with Class 14-18, but I was no better off than any of them physically. Why? Because I tried and I showed every staff member at OCS that I was willing to do whatever it took to get better and to earn my commission. I showed improvement with every PT test, and eventually they stopped saying anything to me. This isn't me being arrogant or patting myself on the back. This is what I know following offline conversations with my class team.
There's a caveat to this, of course. I owed it to myself, to OCS staff, and to the American taxpayer to show up at the physical standard required, so that I didn't add 3 weeks to my time. I should not have had to roll to H Class. That's not the point of OCS, and that is a failure on my part. I should have prepared better, and my lack of physical fitness was no one's fault by my own. No one should read this and take "Oh I can show up to OCS as hot garbage and be totally fine" as the point. You owe it to yourself, OCS staff, and taxpayers to show up in the best physical shape you possibly can. But if you aren't a PT stud, you are not automatically doomed. I saw many candidates spend 6 weeks in H Class improving their run, push-ups, or curl-ups even, and they are, in my opinion, some of the greatest leaders to graduate OCS. Class 14-18's president spent 6 weeks in H Class and is, to this day, one of the strongest females I've ever had the privilege to know. She is a massive asset to the United States Navy. They will kick you out if you fail to meet standards, yes. They won't hesitate. I've seen good people get kicked out as well. But they also know when people are trying, or when they can be helped. You have chances.
I asked my chief what exactly the standard is for kicking people out for the IST. He said, if the command can look at your PT score and reasonably believe that you can pass a PT test after spending time in H, they will roll you. I have never heard of someone showing up to OCS and being attrited after failing the IST.
Don't test that theory. Be as fit as you possibly can be when you get there. Things DO change.
But know that if you're not leading the pack at morning PT, that does not mean you automatically fail. That means you must work your ass off and show everyone there, beyond a shadow of a doubt, that you can and will do better, you will improve, and you have the heart to keep trying even when your class has left you behind.