In theory if you invest in well-diversified and low-cost stock mutual funds, your long-term returns will outpace those of conservative investments like Treasury notes. As desecribed in this article by Rick Ferri,
it might take longer and with more risk than you had anticipated:
Making money in stocks can take a long time. A $10,000 investment in the S&P 500 during April 2000 grew to only $10,306 by October 2011. A similar investment in 5-year Treasury notes grew to $20,920 over the same period. U.S. stock prices must double in value to close this gap. That may take years.