Dr. Hawass and other Egyptologists have recently estimated that there are far less blocks used in the GP then previously believed. The ancient Egyptians built the Great Pyramid on a natural "mountain", thus limiting the amount of core blocks required in the construction.
20 years is easily enough time to construct the pyramid. This would only require 81 blocks being placed a day. With 10,000+ men working to achieve this feat, it is hardly extraordinary.