I don't remember exactly when I ceased doubting whether I could backpack 20 miles in a day. I think it was somewhere in Virginia. By New York I no longer doubted I could cover 30, no matter the terrain. That mentality has indeed gotten me into some tight spots, but the great thing about backpacking is that I always have everything I need to survive strapped securely to my back. I am self-sufficient.
Lack of self-sufficiency is something I struggled with (and still do) when it comes to ultra-running. About 1 year ago I finally gained the confidence that I could complete a 50k no matter the terrain within the cut off. Although I had not worried about a hiking itinerary that included that distance for years, getting to that point in the context of a supported running event was very difficult for me. It was a great feeling of freedom to finally be able to stop worrying about the distance and simply go. It's also fun to be able to sign up for a 50k race at the drop of a hat and not have to really think twice about it.
I feel I am very close to reaching that point with 50 milers. Not that I'm particularly fast or good at them, but I certainly find the distance far less intimidating now. I've backpacked them a few times, and run the distance 5 times in 2 years. I can't say that I feel the same way about them as I do 50k's. I don't feel a level of confidence that I can just show up, no matter the terrain, and count on beating cut offs. But, it is no longer a break-out-into-a-nervous-sweat moment to sign up for one. I've run 2 unsupported 50's in the back country. They have been challenging, but rewarding, and they have helped ease my sufficiency concerns. Eventually I hope to reach a comfortability with them that rivals that of shorter distances.
I am approaching a new plateau, therefore it's time to kick the challenge level up a notch. My break-into-a-nervous-sweat sign up moment now comes for 100 milers. My first one is about 7 weeks away. I alternate between being scared to death about being out there alone and excitement about the adventure of it all. I think I'm ready for the challenge, but just as it was for my first few 20 mile days on the Appalachian Trail, then my first 50k's, and most recently 50 milers, I am still not comfortable and confident I can make the distance within cut off.
But, just as it was for the other challenges: There is really only one way to find out.