Just finished my first 20 miler and feel like dying. I thought I was doing everything correct to prep for it; carbs last night, powerbar 45 min before, plenty of water, pre/during energy drink (Revenge Pro by Champion Nutrition), packet of GU at mile 10, and water during. Had an 'Endurox' shake after I was finished. About 20 min post-ride i am experiencing nausea and a gnarley headache. Any advice? Am I just out of shape or am I doing something wrong?