i vote for loops, but i'm not so in adamant about riding loops that i'd never do an out & back. i've done that too. my first trek this year will be an out & back probably. then hopefully a 5 day one way.
one trip i'd like to do in a couple of years is bike to the train - train to Chicago - bike to a ferry in Wis., then bring the ferry back to Mich, then ride back home a couple 100 miles.
i think that'd be a really cool loop.
do what you can get done and enjoy at the same time.
This is a loop my wife and I did a couple of years ago that might interest you.
It was "Plan C", and how we got there is along story. Plan A was a circumnavigation of Lake Erie. Plan B was a circumnavigation of Lake Huron. We changed plans up in Mackinaw City when the weather turned cold and it was spitting snow in Canada. It was mid October. Come to our bike club meeting next week, and you can hear the "rest of the story".