That's so true.  Up until recently, I had never done any serious running.  Had a treadmill at home and just ran 5k for fun now and then.  Once your body gets used to those shorter runs, you think "hey running's easy.  What's the big deal?  You can always just go a little bit farther."  Then I entered a 10K road race and about 9K in I felt the effects of glycogen levels dropping for the first time in my life.  All my leg muscles complained and my body just felt exhausted in a very hard-to-ignore way.  Turns out different distances have different sets of problems.  You can't just do "a little bit of training" and then go into a IM expecting modest results.  Your only result will be "I had to quit 1/3 of the way in because I just couldn't do it."