Can I legally walk away from my house being that I am so far behind on my mortgage?

If you walk away, the lender will eventually foreclose and it will hurt your credit. Additionally, the lender might sue you for a deficiency (the difference between what you owe and the value of the house at the foreclosure sale). You would be better off trying a short sale for less than the amount you owe and getting the lender to forgive any deficiency. I know some good realtors who can help you.

It depends on the law of the state where you live and the type of loan(s) that you have.

Yes. But you do that only if your mortgage is "under water," that is the loan balance exceeds the market value. You might also want to try negotiating with your lender instead of just walking away. Banks are often willing to pay moving expenses or other amounts of money in exchange for a "deed in lieu of foreclosure," meaning you save the bank the time and expense of going through the foreclosure process by deeding the house to the bank. This also might be less damaging to your credit than a foreclosure. You might also consider a short sale agreement whereby the bank allows you to sell the house for less than the amount needed to pay off the loan. You should discuss your options with a Realtor or a lawyer.

You can just walk away from the property. It is better to short sell the property.