I can't run in my dream

A couple nights ago, in my dream, I tried to run home, and I couldn't. I tried to pick up pace, but something was weighing me down. I remembered that happening, in my other dreams, but it didn't occur to me that I was in a dream at that time.

It's strange that that happened. What could have caused that? My brain could not generate new surround images fast enough? Or that I am really so out of shape my muscles were sending signals back that running was not possible?