Imagine . . . You’ve been preparing yourself for a couple of days now for that 180 km.  On the eve of it you scoff down 250 g of pasta.  On the morning of it you get a really good breakfast.  But, unexpectedly, you get a flat at km 30 (it happened to me exactly in this scenario).  Or, your handlebar stem gets loose.  What do you do? Call your friend with his ute to pick you up after work?
Look at the picture.  This is only what it takes to overcome most problems commonly found on the road: flat, loose parts, offset derailleur screws.

So, what have we got here?

• A light electrician’s screwdriver to tune your derailleurs.  They can turn out to be so pesky!
• clincher unmounters (I assume you have clinchers)
• set of Allen keys (no big ones for pedals though, just the most common ones for smaller bolts)
• an inner tube (make sure the valve length and type suit your rim and CO₂ dispenser respectively)
• A CO₂ cartridge and its dispenser

By the way, in case of a flat, change the inner tube.  Don’t bother patching it, you will have problems later when the patch comes off by itself after a couple of months).

It is a good idea to put all metallic parts inside a plastic bag, or food wrap, to prevent them from getting rusty through rain, or through your sweat if you have a bumbag.