Im joined football in my junior year and Im worried?

Ok so I just tried out for football as a lineman defense, and I got my pamphlet already, and in the pamphlet it said that there are kids that may get cardiac arrests. I personally am not good with cardio, I can only do 20 push ups and weigh 215 lbs but Im physically built and can practically bench my weight. Im just worried, any tips you guys can give me on how to survive all my practices and the two a day