I'm sure people like Double and Cameron can offer much more than me. I'm not even sure if any words from me would count as wisdom, given my short lackluster history. All I'll say is a 50K is less than 5 miles longer than a marathon. While you have to appreciate the extra distance, don't fear it. As with the marathon, restrain yourself early, find the flow, and just roll with things for at least a couple of hours before you even think about worrying about racing.
The biggest difference I found between the marathon and the 50K was the support on the course. Once you're running an ultra, plan on taking care of yourself. There might be some race provided support out there but it is sparse and they won't be trying to get you in and out without breaking stride. Of course, I wasn't running a trail ultra and there are no doubt more changes of significance when introducing that variable.