Your post gave me the shivers! I almost though it was my daughter posting it as it sounds somewhat like her present position.
Anyway, until you have actually started the position, I don't think you can be held to accounts. While it doesn't look good, it would ultimately depend on why you did not honor the commitment. For future employers, if the explanation is a logical one I doubt if it would have much impact.
However, once you have started a lot will depend on the terms of the contract as generally there will be as probationary clause. That clause is there so that it gives both parties the option to get out as sometimes a person is not up to snuff or not the right fit in the corporate identity. Or, it could be that the way the company operates goes against the new employees beliefs. It may have a monetary penalty but would have to be given equally to both parties. It may have one day's notice, or 3 month's notice again, or salary in lieu of, depends on the contract that you signed.
Having said that much, I would point out that all here is purely conjecture and the only real way would be to approach someone in the legal profession for confirmation. Without seeing that contract I would be afraid to say anything further at this point.
SOME PEOPLE TRY TO TURN BACK THEIR ODOMETERS. NOT ME. I WANT PEOPLE TO KNOW WHY I LOOK THIS WAY. I'VE TRAVELED A LONG WAY, AND SOME OF THE ROADS WEREN'T PAVED. ~ Will Rogers