Push a bigger gear, faster. If you finish and think you could've ridden faster you've blown it, if you fall apart before the end you've blown it. In both cases, just try again, there's feckin' loads of 25s. Or head to the Welsh course and cheat your way to a faster time :)
Maybe you need to do more longer effort sessions.
You might be a lot better at going hard for 20min but not able to keep that going out to an hour.