You've been given good advice thus far.
I was emancipated and joined the Army when I was 17. I served from 1992-1995 and was deployed to Haiti for "Operation Uphold Democracy" during the Clinton administration. I did get my high school diploma with a 1.8 GPA, but in retrospect a GED would have sufficed. (incidently after my service I went to college on the GI bill and completed my BS with a 3.8 and my MS with a 4.0...so it's never too late). You may want to purchase a study guide for the ASVAB so you can obtain scores that give you more options for choosing an MOS.
Beyond the 100's of push-ups, sit-ups, flutter kicks, mountain climbers....I found the most difficult part of basic training and air assault school to be the marching.
I would seriously reccomend purchasing a surplus mil-spec ruck sack and practice marching with ever increasing weight for ever increasing distances over increasingly hilly terrain. When I was in the standard was 12miles in three hours with a 50 pound ruck sack. I ended up with stress fractures in my feet (but never fell out of a march or run), and had to take large quantities of ibruprofin every morning just to walk. My feet toughened up for AIT and my permanent duty station (25th infantry division), but I wish I had "tough feet" prior to basic.
Ranger school will be even more demanding and you will likely have to march twice that far.
Good luck with your career goals, and if you play your cards right, you could have a retirement check in your late 30's or early 40's and still have time for a completely new career. I often reflect and wonder what my life would have been like had I stayed in for an enlisted career.
gl bud!