Long Riddles and Answers

Question: I'm made up of 12 completely different letters but you can type me with just 1, what am I?

Question: Like a middle pillar I stand among 6, all 9 of my characters make me the largest to exist.

Question: A chicken gives meat and egg while cow gives us milk. Find another who gives us all these?

Question: Until I am measured, I am not known. Yet how you miss me, When I have flown What am I?

Question: I can run but never walk. Wherever I go, thought are close behind me.

What am I?