If I started earlier at age 24, I would only have to contribute approximately $60,000 a year which is well within the range of 2014 median pay for an economist of $96,000 according to the Bureau of Labor.
It's unlikely that you will be making the median salary when you get out of the university. For most professions, salary start low compared to the median and take years to grow above it, unless you work in a big firm in a very big city and work an extraordinarily large amount of hours. Since you imply that you will be living on 20K$ a year, I assume that you will not be working in a very big city, otherwise your living expenses would be much higher.
And if you can live with 20K a year, as you are implying, why do you need 2M$ (in real terms) to retire?
I think that if you want to maximize your chances to FIRE, you should focus on studying in a field
that you will enjoy (this is extremely important for long term success) and that quickly yields good employment opportunities. I don’t know where you live, but where I live there is not a lot of opportunities for pure economists. Most of the time economists also have other qualifications such as the CFA designation, a MBA or some form of accounting training.
Finally, I suggest that you do not follow a rigid plan, but rather a set of principles. You can be sure that whatever your long term plans are, they will not happen as you planned them. Success happens to people who are flexible, open-minded and that adapt quickly to opportunities.
I’m not writing you this to discourage you of reaching FI. I have no doubt that you can do it. However, having more realistic expectations will likely yield better long term results.