I ran a few but the worst experience I had was in Rehoboth Delaware. It was the innaugural race. 14 degrees with 20mph winds and 40+mph gust. With windchill it was in the negs. The first half the wind was at your back and you were somewhat sheltered from the wind until you got on the main roads by the ferry. The halfway point was on top of Cape Henalopen and on the way down the wind was in your face. (mind you I was bundled up on my upper body but wearing shorts down low.) As you retrace your foot steps you know it is going to be a loooong 13 miles back because of the blistering cold. But as you start seeing the signs for 5 miles to go, 4 miles to go, etc... you know the experience is almost over. I crossed the finish line at 4:35 and was happy due to the fact I decided when I got there to not race it because I would not enjoy it.
The best experience was the same one because I actually had a good time staying a bit slower than my normal pace and just enjoying the scenery. The cold was no fun and I was a bit hypothermic, I believe. But it was an experience.
I usually run them around 3:30-3:40 depending on the course. I am a bit more interested in Ultras now. I have done some 30 mile runs but I am training for a 100 miler hopefully to complete in the fall (depending on the weather up here)