you can pace yourself a couple of different ways.
1. If you have a power meter then depending on the length of the race you can use a % of FTP.
2. If you do not have any power meter use perceived rate of exertion. Unfortunately to get a solid and accurate value takes a lot of experience.
generally people go too hard in the beginning and fizzle out at the end. usually you start a little easy and go harder and harder.
try making an effort at the 1 hour mark to really kick up the intensity. it is going to be trial and error at first to find the effort you should be doing.