From many years of taking people out for their first experience of the trail, I can tell you that the #1 comment I get is: "it was harder than I thought it would be."
Many people work out, or exercise, and everybody walks. But backpacking is a particular sort of physical challenge. Most other athletic activities do not adequately prepare a person for the physical demands of backpacking. People are very surprised when, even if they are an avid runner or biker or tennis player or swimmer, that backpacking is still so hard.