Discovery: 20 miles is a difficult run...

So I've been training for my first-ever marathon.  I've talked about that a little.  It is now less than 2 weeks away!  With that in mind, I geared up for my final long-distance training run last Friday.  20 miles.  I learned some funny truths about myself on that run:

Funny:  How my mind works when I'm planning for something like this:
"20 miles...lets see...where to go?  Forget about inclines.  They are not a problem...just short bursts of pain amidst a much bigger pain.  Won't even feel them.  Guess I'll just run down the river to the dam and back.  Hmmm...not quite 20.  Yup, there we go...a nice little loop before reaching the house.  Simple.  Perfect."

Really Funny:  The fact that I did actually look at the vertical profile of the path before I concluded that it would certainly be a nice run. 

Extremely Funny:  Having looked at the profile, I did not note to myself that it would be downhill for 11 miles and then uphill for 9 miles.  Ha ha!  Ha.  *shaking head in agony*

Truthful:  I could certainly, and almost easily, run any one of the hills on my planned route in a run under 10 miles and still maintain a decent pace (decent for me anyway).  In fact I could probably even run several of those hills in a 10-mile run and not have tremendous amounts of pain.  (See how my mind has already forgotten the severity of the beating I took on Friday?)

Also Truthful:  I WALKED the biggest hill that slapped me in the face at mile 17.5 .  And I don't like walking when I'm on A RUN.

Painfully True:  Beginning to run again after running 17.5 miles and then walking 0.5 miles uphill is extremely difficult...but possible...yes, and very painful.

Thankfully True:  I still had the mental stamina to hit the 20-mile mark running; despite the extreme gravitation I felt toward cutting that final loop short.  At that point the thought process was something like this: 
"If I cut this run short I will not do the marathon...after all this training and time invested.  It's too late in the game and too close to the finish to do that.  Enduring pain is part of the process that I expected.  So, which street do I turn on?"