When Eastwood filmed The Eiger Sanction in 1975, he took his psychical abilities to the limit. Before the shooting, he realized that his stunts were too dangerous for his stunt double to perform and decided to perform them all by himself. Eastwood then underwent exhausting training, including climb courses, mountaineering, and rope climbing. While today all of us are accustomed to special effects and CGI, the film had no special effects and Eastwood did all the dangerous stunts by himself.
Staying fit is certainly a commitment. Besides doing the physical training, there are some other factors to consider, like nutrition