Completely agree. We probably need more than 20 years to build and test the infrastructure and logistics to send someone to Mars (little things like the actual vehicle to get there is barely more than a pile of concept drawings). And that's assuming we don't continue our history of changing NASA priorities completely every couple of years. Maybe in 100 years.
After a whole paragraph describing how difficult it is, this seems rather optimistic. Give or Take? Give another 100, I'd guess.