Peaking just in time?

Thursday: 8 Miles, easy pace
Saturday: 20 Miles, easy pace, not-so-easy run.
Tuesday: 4 Miles

How could I fly through my 13 mile run last week feeling as though my legs could propel me through a concrete wall, but then this week I trudge through my 20 feeling as though I were running through knee-deep wet cement? 

It didn't instill a lot of faith in me that the upcoming race is going to go well.  I'm hoping that I peak soon.  The next few weeks are nothing but 4-mile runs with an 8 this Saturday.  Fast running coming soon.

It's go-time people.  Help me send my marathon finisher medal to someone who is aggressively fighting cancer.  I need your assistance.  Click on this link: http://pages.teamintraining.org/dm/pfchangs12/mnortonnpu

Thank you!