You want to be in the best shape you can be in when you show up for matriculation. You will want to start running regularly now. You don't have to be a sprinter or a marathon runner, but you want to make sure you can keep up without struggling. Work on pullups and pushups too - you'll be doing lots and lots of pushups. If you don't show up in great shape, you will end up in great shape by the time you breakout, but life will be easier if show up as physically ready as possible. You will be tested physically, mentally and will be under a lot of stress in every way, so being in good physical condition when you show up will help a lot. Don't worry now about how much time to allocate to academics, etc. You will figure that out when you get there and you won't have much free time at all. For academics, probably the most important thing is don't try to study in your room. Find a quiet academic building.
Show up in shape, be prepared to give your best effort at everything you are asked to do, be humble and don't take anything too personally. Don't be the one that falls out of runs or has made up injuries all the time to get out of PT. The ratline is a time-tested process. It will not be easy, but if you truly do your best, trust that the process works even though it's going to really, really suck at times and don't take things personally you will be fine.
You have chosen a tough, but potentially very rewarding path. Best of luck to you.