Decks, Patios, Porches and Docks - Free standing deck / cabin floor

I am planning to build a free standing deck eventually to become a hunting cabin. My plan is to have the cabin 24 ft by 20ft. I am planning to have 4 - 6x6's posts on the outside perimeter along the 24 foot side with the distance between the posts to be 7.5'--8' --7.5' respectively. To both sides of these 6x6's to level and lag bolt a 2x8 on each side to serve as beams. My question is can i place another row of these at the 10foot mark on the 20 foot side to stretch across the 24 foot length and then lay a 20 foot 2x6 joist across the beams on 16 " intervals. Is this too far of a span for a 2x6 floor joist.

Should i plan on putting the 24 ' beams on 5 foot spacing along the 20 foot side with 2x6's as joists. suggestions please.