Not personal experience, but I have talked to several guys while at ABN school who had just come off of Sand Hill.
All advice given thus far gets a big +1
In addition, EMBRACE THE SUCK!
If you don't make it into the Rangers, then yes, Big Green will do with you what it likes--so prepare thyself in the ways prescribed!
There are people younger and older than you, I met several with degrees, which is increasingly common. Your age will not matter--let your word and deed represent you, and to each in the same way. In other words, don't fuck your buddy!
Barracks--you will get sick and be miserable. Happens because your buddies are from all over the country. Suck it up (you'll hear that a lot too...) unless it is life threatening or permanently damaging. HSLD gear, besides attracting negative attention, might get stolen (the intent is for you to learn to endure suffering, which such gear shows you are averse to).
Do not attract negative attention; I would argue one would actually do this more by NOT volunteering. SO, it would behoove you once more, to help your buddy.
ABN is reportedly really low key compared to basic. As stated above, you will be heading to RIP, so invest in your future by not slacking off.
The only way to get better at running is....to run. If you tire in the middle of a course, practice speeding up at this time! If you suck at hills, run hills! Google is your friend for safely and effectively planning training for your needs.
Hmm. What else...you can do nothing right, which is part of the breaking you down thing. And if someone says a certain activity is prohibited in aiding reinforcement (AKA, "getting smoked") that just means they do it when no one important is around.
AND, FROM PERSONAL EXPERIENCE: The First Sergeant is called "FIRST SERGEANT". Stand at the position of parade rest when speaking or being spoken to, if you value your life.