Once upon a time...
As far boot goes, they expect people who are as fit as Rosanne Bar and work your way up to meeting your requirements. Find out what the requirements are to graduated and work at that. Running 5 straight miles will do you no good if you can't get your mile/1.5/2 mile run in time. It isn't hard, I passed the same requirements when I was 28 as when I was 18 (even though I weighed 50 pounds more).
In my boot, with exception for your actual test, effort was more important then ability. If you could only do 5 push-ups, and the guy next to you could do 20, they made sure you both did your max. They won't go easy on you for being fit, you will just have to do more. Not bad thing, but don't expect a free ride. Sadly, I worked with a guy who was a body builder before joining. He had to drop his weight to join, he was over the weight standard
Put 100% effort into everything you do in boot, and you will be fine. What comes after boot is a crap shoot. I consider myself to be a successful person, I have goals and I work until I meet them. Example, I worked from 7 AM to 7 PM today, but only charged for 8 hours. I had a goal I wanted to meet, and I met it (4 hours late due to weather). I also made E6 in 5 years. But I was treated like shit in the military. I don't kiss ass, and that was what they wanted, so I was always dumped on. They want shit to roll down hill, they do not want you to stand up for your people. Honesty isn't the best policy, finding blame is.
So I left, and now I make a lot less money (most people do not realize how well they are paid in the military) but I am happy.