We're talking longer than 10 years here. A better estimate might calculate the average 10 year return for every 10 year return period, and the standard deviation of that same data set. And then do that calculation for 30 years. I believe you end up with a higher average and smaller deviation at 30 than 10.
from 2006-2015 the SP500 had a geometric yearly mean return of 7.25%.
according to http://www.usinflationcalculator.com/inflation/current-infla...
The average inflation for that period is 1.9% so you get roughly 5%.
Still pretty decent.