How do you do what you're doing and how do you do it for as long as you do it?

So here's my big question - how do you do what you're doing and how do you do it for as long as you do it? What? This probably sounds crazy but here is where it started. At the lunchtable the other day I asked a fellow teacher about his teahcing career and he said he had been teaching for 16 years, another then another teacher said, oh yeah I've been doing this for 20 years and then another for 14 years. I'm sitting now at 7 years and I'm asking myself this question: how do you do what you're doing and how do you do it for as long as you do it? I'm kinda amazed that I'm still teaching to be honest because I could never imagine myself doing anything for that long a time but then I look around and see myself in the other teachers when I asked how long they had been teaching. Sure, I know you do it because you have to work, right? I mean we have to work to make money to afford all the cool stuff in our lives, but how? Do you make a conscious decision and say I have X amount of years until retirement or until I have vested money or do you say, that's enough I'm moving on to something different. Of course I might not be best person to ask this question seeing that in college, I worked at about 15 different restaurants and bars. It was this weird thing where I just knew when it was time to move on- either I got up enough gumption (sp?) to move on or places went out of business or the management changed, but I just knew when it was time to move on. Does that change when you grow up and get a real job? Or do you just tweak your job and try ot make it new and interesting? Everytime a fellow teacher says oh yeah I've been doing this for 18 years, the hairs on the back of my neck stand up? Am I some sort of snob? Why does the thought of doing something for 30 years give me the cold chills? It's not really about whether or not you like your job or not, I really like my job, ALOT, but can I really do this for 30 years? You say, change it up, well hoping to next fall to take some classes but still, I think, can I do anything for 30 years? What about you?