Just a quick thought about improving how the checkpoint feature currently works. When it first came in, you set one checkpoint, and had to be careful about when you set that checkpoint, as you only got one.
It now seems as though it has changed that when you return to your checkpoint it removes it, so you have the ability of setting it again at that current spot, or having it in your back pocket for later. Whilst I am in the camp of having the ability of having the checkpoint again for later, the current way has the drawback of users possibly forgetting to set their checkpoint upon going to it.
Some people I have talked to have wanted an option of having multiple checkpoints, but the easier answer seems to be just adding another button. Having a "Set Checkpoint" and "Go To Checkpoint" button, where the "Set Checkpoint" button sets overrides your current checkpoint should fix any issues. I made a quick mock up of how it could look in photoshop.
I put the "Return To Start" flag back above, where it originally was, as it looks a bit cleaner there. "Set Checkpoint" as the current + button, and "Go To Checkpoint" as a tick.
Anyway, just a suggestion.