Not too sure what the hardest workout I've ever done is, but the hardest thing I've ever done would have to this one half-marathon I did in September. I'm a decent runner and I love distance running, but this half was insane. It was very hilly, but I was still averaging about 6:20s for each mile, even with the hills. Then, at mile 11, came the biggest, steepest hill I've ever had to run up. It slowed me down to a 7:35. I was crawling up it and my quads were burning like hell. Then I had to finish the race feeling like I was going to puke (my stomach was upset the whole race too and that made it worse) and with quivering legs. The hardest workout might be repeat miles at 5:30 pace, 5-6 of them.