I believe those figures are based upon the average historical growth in equities (somewhere around 7%).
Extrapolating, a conservative investment in an index fund should generate about 7% annually, doubling your principle investment about every 10 years (Rule of 72). Unfortunately, I don't think the stock market's been anywhere near this level on either the current 5- or 10-year return rate.
So, why not put the money into the S&P 500 and be done with it?