Not quite sure what is happening...Can anyone offer any explanation and/or solution?

I have been running for the past 5 years 4-5 times a week, 4-5 miles a day, and every now and again I will hit a physical wall where in which I can only run a mile or two for a period of time that could measure a couple or days to a couple of weeks. During these times I have to often stop after the firest 20 minutes into my workout and walk for a while and then try and finish my run. Or there are time where in which my body and legs are so tired that I have to cross training on the elliptical machine. I hydrate, rest, add protein to my diet, but to no immediate physical response. I am currently dealing with one of these periods and the last time I had a really good run was last Thursday. My last three outing including today was awful. Any suggestions as to what might be going on and how I can break through this malaise.