-du sitzt nur noch die Zeit hier ab un träumst du vor dich hin , und du grübelst und ganz plötzlich kommt das Lied dir in den Sinn = Zeit absitzen means, that you just wait for something to be over without enjoying it the least. vor sich hinträumen is basically daydreaming, so you wait for the time to pass by, you daydream, you ponder and then this song comes to your mind.
-Heute heißt es schnell was essen, sonst kommst du nicht mehr mit, dein Magen wird´s dir danken , du hälst dich mit pillen fit = tricky one:) "Heute heißt es"= today it's ont the schedule. So today you have to eat something quickly, because other wise you run out of time, but your stomach will be greatful, you keep fit with pills. I guess this mains, that you have hardly time for eating and especially not for eating something healthy, so you take some vitamine pills as a replacement for better food. but maybe I am too naive and the person is taking something stronger, than vitamine pills to keep himself up to the work that has to be done.
-Doch plötzlich geht´s nicht weiter , du merkst es läuft nicht mehr.- = roughly: but suddenly you cannot go on, you notice, that it does not work anymore (the lifestyle I guess).
Hope this helps a little :)