How realistic is it to think that a person could travel the world by foot?

Obviously you might have to go overseas sometimes but what i mean is, just say you started of in the middle of europe one day then you started walking towards asia. you dont have any money except the money you earn as you p through towns doing various jobs (say being a strawberry picker for a day or 2). once you've done this you move onto the next town... maybe you could spend time working on a cruise ship. Anyway you reach asia, get across to canada somehow, walk down to the bottom of south america... im sure u get my drift... the question is: Is it realistic to think that someone could someone spend their life like this? ?Has anyone done this? Or know anyone who has?