Like you said, that is a very conservative estimate, but entirely accurate. If we experience a horrible depression which gives me no investment returns for 20 years, I still will be able to retire at age 43.
The rule of thumb is that you need (Current_Expenses x 25). This is a 4% withdrawal rate, and should last you forever. If you want to go even safer (the Trinity Study suggests that 4% is plenty safe) go with 3% and multiple your expenses by 33.
Implication: You must save 25 dollars or so for each dollar you spend. So you go out and earn an extra 25 dollars, or figure out how to spend one dollar less.
Two excellent suggestions.
1) Very good point, and I will be changing the name of this post. I named it Rational Financial Planning Overview as a counter to another discussion post Preparing for a Rational Financial Planning Sequence, the ideas which I disagree with. I did not expect for my post to be taken as well as it has (a very pleasant surprise) so I will be changing the name to more accurately reflect its contents.
2) I did not think of this, though I should have. Cutting your expenses and saving/investing the greater majority of your income is applicable in any financial endeavor. I will be adding an edit to this effect.
Thank you very much.