The first time I ran 100 miles, I was just happy to be there, literally. I was the only friend I knew that had done it and I got injured in the final weeks of training, putting doubt into my ability to finish the race. As a result, I kept a pretty positive attitude the entire time and just enjoyed the journey and worked as hard as I could. My second hundred miler was my worst, because I had become a decent ultra runner and wanted something way more than "just finish". I had a vision of how my day would go -- my nutrition, my ability to move in late miles, etc.... My body went south, then the weather, then my attitude. I walked the final marathon. My sister recently told me that was the only time she's wanted to choke me as a crew member for all my 100s. The big difference was the expectations I heaped on myself and communicated to others about how I was going to do in that race. I let those external factors influence me and my attitude. I've since realized that 100s are unpredictable and the best you can do is take it in phases, one aid station at a time.
Attitude Is Everything
Building on the above, I have taken the approach that keeping a positive attitude makes all the difference. 100 miles is a long time and things will go wrong. If you are trained, and experienced in particular, the things that go wrong are almost always not the thing you've considered. You choose whether you let that influence your attitude or take it in stride and keep moving. This is a crazy endeavor and most of us are just fortunate to have enough health to even complete it. Without a good attitude, you cannot give your best at anything you do in life as excuses and disappointment will creep in.
Focus On What You Can Control
The best way to overcome expectation and attitude is to focus on what you can control. You don't control the weather, a very common reason for poor attitude and results. You don't control your pacers, crew, other racers, etc... You control the decisions you make and when you make them. Stop to take care of problems that come up before they become major issues.
Grit And Mental Toughness Can Carry You
Long after your body has gone, your mind can carry you. My buddy ran the last 30 miles blind because he just kept moving and focusing what he could control. I paced a runner that ate about 400 calories over 10 hours; he had nothing in the tank, but he kept moving. If you are committed to it and you decide you are going to finish at nearly any cost, you can do it. It isn't easy and it will test you down to your core, but there is a path through the pain and the disappointment if you don't quit.
You Can Do More Than You Think You Can
This is famous Ken Chlouber mantra, and he's right! Most people I talk to are floored at the concept of running 100 miles when they find out I do this for "fun". I always tell them, "if you are prepared for the challenge, you can do it". Honestly, it is more about avoiding problems like dehydration, blisters, bonking, etc... and keeping an attitude of grit and determination than anything else. Sure, fitness plays a role and I am not discounting that, but you don't have to train 100 mile weeks to run 100 miles. You simply have to be willing to go through the pain and fatigue.
Focus on Small Goals and Gain Momentum
The simplest way to approach a 100 is to not think about it! Every time I think things like "50 miles to go" or "15 hours left" or "I'm so tired and I am only half way done" or "that climb at mile 60 is going to suck", I get overwhelmed by the challenge and remove myself from the moment I am in. I simply remind myself to focus on getting to the next aid station or getting over that climb and celebrate that. On a big climb, I give myself a verbal congratulations after some interval (depends on how large the climb is). It is impossible to never think about it, but I keep those thoughts to a minimum with self talk.