I'm glad someone has caught on! For me during a marathon, the early to mid miles would be Biohazard and Machine Head because of their mix of melody and thrash. That makes it easy for me to conserve energy. Then at mile 18 or 20 when I feel like I'm hitting the wall, I'll put in Slayer, Hell Awaits and Reign In Blood. If I were to listen to Slayer too early in a marathon, I'd bonk much sooner. For a Half Marathon, Show No Mercy, Hell Awaits and Reign In Blood get on my playlist, plus some from Haunting The Chapel.