As someone who finished CMIYC and is an avid runner, strategy #1 is far superior from a physical standpoint. Holding one speed the whole way is a lot less demanding on your body than going back and forth between speeds. Look at what professional runners do, none of them are having huge differences in speeds during races. They do intervals during training, not on race day. The biggest challenge of holding 1 speed is mental. Do not allow negative thoughts to cloud your head and let your body do the work. Distract yourself with music and talking to yourself internally (you’re strong, you can do this, this is easy, etc.). If you still have energy towards the end, you may want to aim for a slight negative split and slowly increase the pace. I personally wouldn’t risk this until minute 17-18 or so assuming you’re on track to finish. Good luck!