Which is faster, going all out then burning out, or steady pace all along?
I don't have the time to give a full answer at the moment, but I'll upvote a full answer and delete mine. The short incomplete answer is that you're better off pacing yourself relatively evenly. The reasons are both physics and physiological. The physics answer is that drag increases nonlinearly with speed so at higher speeds you're using more of your energy overcoming drag. The physiological reason is that when you go over threshold, you get tired (that's why it's called a threshold) and the time needed for recovery is exceeded by the time you're able to spend above threshold, so the net effect is that your average power output will be lower. 

These two effects, the physics effect and the physiological effect, mean that unless your ride is shorter than, say, a minute, you're better off pacing (nearly) evenly.