Does marinading preserve food's freshness
Marination is a process defined as soaking something in a flavorful liquid to impart that flavor into the object being marinated. Marinating is really only working with the outermost layers of the food and is normally a quick process (hours). The liquid in which you are marinating is the real difference maker. A highly acidic liquid CAN have an effect on the preservation of the food and can also have effects on the texture (think ceviche). However, in most cases marination is not used as a preservation method, that is primarily done through curing in a high salt environment where moisture loss is promoted to prevent bacterial growth (i.e. charcuterie, bacon, etc.). 

But as for the food safety portion of the question, as long as it still smells ok and has been kept in the refrigerator for that time I believe you should be alright. 

http://www.cookinglight.com/cooking-101/techniques/cooking-class-marinating-00400000002287/