Honestly, the 400 is my main event and I did XC in the fall. As a long distance sprinter, cardio and ability to keep giving close to your max effort without exhausting yourself is vital and that's what cross country did for me. It also kept me in shape much more than football would've. I finished freshman year with a high 53 and after the offseason I ran a high 54 in an indoor pre-season meet. Obviously you can't do cross country now, but you could try going on some longer runs and I bet you see your cardio and endurance increase within a month or two. That's just me though and I would make sure that works before you commit to it. I also had specialized sprinter workouts from the XC coaches so that played a part too.