I like your shadow idea. I originally wasn't expecting them to jump threw the walls or catch me if I was on the other side of the hill. I would recommend making the shadows distinguishable from each other as I step one and expected to die, but it instead let me ride on it. Don't know if this was intentional or not.

The controls are very responsive despite the character moving quickly. This will be useful for making sudden changes to your direction when you're about to run into your shadow.

With the current set of levels, it appears that the subsequent level builds upon the previous on in some way. it will be interesting to see how later levels are designed.

I think the delayed start between each shadow will be helpful in later levels, and the repeating loops of the shadows gives the player a chance to refamiliarise how they completed the run in previous stages.

The shadows having slightly different effects is quite interesting: the first shadow would reset the level on collision, whereas the second shadow could push the player and be used as a platform. Depending on how later levels are designed, it can lead into interesting scenarios where the player has to avoid the first shadow and time their platforming to use the second shadow (or avoid the second shadow to avoid being pushed off).

Overall, I think the controls are fairly solid, and that you have a solid base to build from.

Very very interesting concept. Will follow its release!