Changing careers at 16 years into a job, is it realistic?

After 16 years as an accounts payable clerk I am thinking about moving into a different field. I would rather hear the honest version than the polite one. I have good customer skills and I am comfortable with spreadsheets. How did you start?

Thanks for writing this out. I went through something close to it 11 months ago, and it settled down once I kept a simple log of every call and form.