Brosnan would surprise people in the role I think. He has that crooked, mischievous smile and that twinkle in his eye that made his Bond fun to watch, and he isn't untalented. It would be a different Doctor certainly, probably a bit more 3 and 6 than we've seen in recent incarnations.
You realize there is a thing called acting right? They film an episode over the course of a couple weeks and they aren't REALLY running around constantly. The running is like 5 feet into and out of frame most of the time. Some of you people act like anyone over 40 is practically crippled.