The game was done well, it was fun and above all the graphics, taking into count that it is a prototype, is developed really well.
However, if I have to be honest, this game has given me many twisting ideas:
In my opinion you were unable to give the idea of an "infinite race" in the right way. Every time I lost, it just seemed like the level was starting all over again, without giving me a choice, nor time to process what happened, and that doesn't seem good design to me. You have to give the impression that you are traveling in an infinite world and give the player feedback on where to go wrong, all this at the same time and without interrupting the flow of the game, and even if one of these things is done wrong or, even worste, is missing, the illusion of infinity would break. And that's why i think you have chosen a very delicate idea.
Let's talk about lives: are they really necessary in an endless runner like this??? A game over is the best way to stop the flow of a game (duh. They are made on this purpose) and, as mentioned before, stopping the flow of the game breaks the illusion that this world is truly "infinite". Maybe you can try to punish mistakes by slowing down the player...
Try to give some spyce to the whole thing: add some obstacles, arrange them in a different way, adjust the difficulty, who knows, try adding a story, maybe it could work. The game, right now, is too monotonous and in the long run it's boring.
Most importantly, the sounds: they are important to give atmosphere to a game and to communicate to the player what is happening. But I can understand that maybe you didn't have time, or maybe (I hope this is not the case) that you thought "this is a prototype: it dosen't need sounds".
Don't get me wrong, I enjoyed the game, it was beautiful and all. I just wanted to tell you what I think about the game and give you some feedback.