Yes the listing agreement is a contract so if it has an expiration date of March 2015 then that is when the contract expires. Each state sets their own laws and regulations regarding real estate transactions. Without having the specifics of the contract to read it is impossible to tell you what your liability might be. Most residential listings are for an Exclusive Right to Sell which means that during the terms of the contract if anyone sells the property then a commission or fee is owed to the broker. In most cases there is also a time frame where after the expiration of the contract if the property is sold to anyone who was shown the property during the listing period then a commission would be owed. Some listing contracts will have a clause which allows seller to terminate the contract prior to the expiration however that usually involves a penalty or fee which would be spelled out in the listing agreement. As far as enforcement of the agreement it really depends as each area and circumstance is different. The only way to really know is for you to read through your agreement very carefully and see what you agreed to.
If I were you I would discuss your desire to cancel the listing with the listing broker. Hopefully they will agree and you can move on. If so then you really need to assess the situation before entering into another agreement. It has been suggested that you have an appraisal done to determine the value. While you can certainly do that it is not normally necessary for a typical residential property. If the property is very high end or unique however it might be worthwhile. For the typical residential property a competent agent will be able to provide you with a competitive market analysis that should give you a good idea of what the property is worth. Pricing the property is one of the biggest mistakes people make. The average person thinks the agent sets the price. They don't the owner does. A good and reputable agent will provide market data and advise the owner. A real professional will be very honest and possibly turn down a listing if the owner's expectations are ridiculously high. I have seen some agents that have a good way of dealing with such owners. They will present them with a listing agreement that goes for 2,3,4 or more years and tell the owner yes you probably can get that price however this is how long it will take. Personally I would never enter into a listing agreement with a duration of more than 180 days. Each market is different but for me 90 to 120 days would be more reasonable.
I have to ask how and why did you choose an agent that is located 90 miles away? I am not familiar with the area so it may well be that they market within that distance.