If you visit ethstats you see two cards named AVG NETWORK HASHRATE and AVG BLOCK TIME, How can i get them in geth?
Geth doesn't deliver the blockTime directly.
I've written a simple PHP script that calculates the average blockTime and difficulty: https://github.com/3lue/Average-Blocktime-Difficulty-Calculator
Maybe someone else knows how to get the average hashrate.
EDIT: I've found following post: https://ethereum.stackexchange.com/a/11382
So you should be able to calculate the average network hashrate with this formula (output in GH/s):
(difficulty / blockTime) / 1e9