Without experience, your adrenaline is your worst enemy. I was amped, feeling good and ready to break 2 hours in my first half marathon (about a 9:09 pace, close to my training times). Feeling stoked, I ran the first mile in 8:10. The second mile in 8:17. The third mile in 8:11. I started dreaming of a 1:55 half; I should've been bracing for a nightmare instead. As you might expect, the wheels eventually came off. Pacer after pacer started passing me, I was doing stretches of walking by mile 11, and I crossed the finish line in 2:01. Cruel—and a lesson learned. Trust your training, and pace yourself.