Tired Arms

So last week I ran 7.5 miles for my long run. After I was done I felt pretty good. My legs weren't tired, my heart and lungs felt great, but my arms were tired. What!!! I try to shake them out every once in awhile, but I guess I need to do a better job with that. Perhaps I should lift weights! My arms are wimpy. Is this just me? I wonder how my arms will do tomorrow on my next long run.