Part of what you learn during INDOC is dealing with the unexpected while sleep deprived, a trait that you will definitely need during your maritime career. Some may disagree with me, but I think specifics are better left unsaid. My 2013 DS did not even tell my 2021 DD anything about INDOC other than that she would be fine.
The hardest parts for both my kids was sleep deprivation, heat, and humidity. Their best advice is to get good insoles to transfer between your various pairs of shoes because you will spend a lot of time on your feet on hard surfaces.
Show up in shape and the physical part will not be that hard.
You will do a lot of marching, a lot of classroom stuff, and a lot of memorization.
Be ready to take placement tests for math and English which are usually on the second day.
Be humble and help out your peers.
Bring your sense of humor. Find small moments of joy within the hard work and tedious days.
Remember it's only a couple of weeks. The hard part starts when classes begin later in the month!