There are a number of ways to monitor your performance and tools available to help you do so. Since we’re all individual it’s normally better to try and gauge yourself against measurable data as opposed to monitoring your overall performance on a final outcome – for example use things like time, speed, heart rate or power to control your tempo and pacing as opposed to saying something like “I want to finish in the top 30” which is out of your control since you don’t know the ability of most of the riders there.
You can normally get an idea of how long an event will take from looking at previous finishing results. If you do this then don’t forget to take weather conditions into consideration and make sure that the course is still the same so you’re comparing like-for-like. You’ll generally get a good idea of what your average speed is from training.
Provided the terrain is similar in the event to what you are used to then this is a good way to keep track of your overall ride. Such are the delights of modern technology it’s now also possible for your nearest and dearest to monitor your performance in real time as well on sites like mapmytracks.com. The only thing is you may get a phone call mid event if you’re caught at the feed station too long!