Nice idea, but needs a lot of work.
The reaction time while racing normal races is simply an impossible feat once you've gotten to a certain speed. You just need to run forward and hope that you won't run into anything.
The advertisement links giving you loads of money breaks the beginning of the game. I understand it's how you generate money but try to space it out or have the rewards be less.
The pacing all around is off. This is one of the most difficult parts to get right, but it should have a bit more attention paid to in the future.
The ninja is seriously overpowered. There is absolutely no challenge while playing as him.|
(Well, until you get to the later stages, but that is more because the draw distance is too low to see what is in front of you before you hit it; allowing the other racers to speed right past you)
The wizard's attacks are annoying on marathon races. He's fine on other races but on marathon in which every bit of stamina is important to keep the game from going at a crawl is a bother.
Animations are a little bland as well as the graphics as a whole but I'll give that one a pass. It is a free web game after all.
The music is repetitive and boring. That is all to say about that part.
And finally, the marathon. The stamina mechanics simply don't work. As soon as you're fast enough to overtake the enemies and put a nice buffer between you and them, it becomes a rather boring game of run, stop, wait, and repeat. The 200 dollar daily races won't cut it if you want to upgrade your character after a while, so you're forced to do a marathon a day in order to keep your daily revenue at a point at which you can upgrade your character.
The idea is great, but the execution is poor.