You can't use the mileage of the total Tour for calculating the distance over which Lance can extend his lead. Instead, you have to use the opportunities Lance had to build on his lead.
In this tour, there were only really 6 opportunities for Lance to gain time. The team time trial, the 2 individual time trials, and the 3 mountain top finishes.
The three time trials are about an hour, so that is three hours. The 3 mountain top finishes take about 30 min each, so that is another 1.5 hours.
So...you have a total of 4.5 hours during the entire Tour in which Lance can put time into Jan, or anyone else. This is closer to two marathons...which puts a 6 min Lance TdF win roughly on par with a 3 min KK marathon win.