My first several centuries and first 200k were on Worksman Industrial Cruiser. Moral: It's not that hard to find a bike you can ride a 100 miles on.
Get a bike that's fun to ride. If you don't know one derailleur from another, you've got no clue if the "better" one is worth $5 extra or $500 extra or what. So at some point, you've gotta pick something and go with it and it might as well be fun.
Don't be surprised if you have to get a new saddle for longer rides- you'll find out when you start riding longer.
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"be careful this rando stuff is addictive and dan's the 'pusher'."