The 1929 figure that you are quoting is seriously misleading in two ways.
The first is that it only tracks the price of the stocks. It does not track the dividends that you (presumably) reinvested. Which effect would earn your money back much faster.
The second way it is misleading is that the Great Depression was a sea-change in American business. Established companies went under. New companies became cultural icons. The result is that a lot of companies on the DOW in 1929 died protracted and painful deaths, while there was a lot of growth in small-cap stocks.
A study in the early 90's at the University of Chicago found that if you had invested at random in January of 1929, and then randomly reinvested dividends, then from just after the end of 1937 onwards you were in the black, and from the end of 1944 onwards your annualized compound returns from the start never again fell below 8%. (Figures drawn from a table included in How To Buy Stocks by Engel and Hecht. Highly recommended.)
Doesn't that sound a little more promising? Even if the next few years suck, your 20 year future should do OK as long as you use some common sense.
Cheers,
Ben