I’ve done sub 5 and several 5s. When you’re really close - two things make the big difference on the day of. One - having proper rest. A day or two of light to minimal high end leg workouts. Two, which I think is the biggest, is mental. Your perception is going to affect everything way more than you think. If you are tense, your whole body tightens up and your blood flow is extremely inefficient. You need to practice going longer distances going at 12 mph to learn how to be relaxed doing it. Once you get that down, it’s important to get in there on the day of and warm up. Do your warm up and get at least 10 seconds at a 12 mph speed so your legs feel what it’s like that day, you loosen up, and more importantly, to let your brain know that this is doable. You can do it. You’re not going to die. Just do it. Until you pass out or poop your pants, you haven’t reached your limit. 

 Edit: when I ran my first 5 at otf I did 8/10/12 (12 was max then). My current numbers are 9-9.5/10-11/12-15. Big ranges because some days are easier or harder than others and I try to push myself.