Motivation, wherefore art thou?

I'm on a quest for the amateur runner's holy grail. The marathon. I've been training for nearly three months now, have finished a 20K race in just over 2 hours, and have reached a max distance of 18 miles. So far. And what else have I done? Not laced up my shoes once this week. It's Wednesday night. Now granted, I did just return from a business trip at 10 o-clock last night, and did have to get up at quarter to 5 the morning before to make my crack o' dawn flight. So, I am tired and rightfully so. And today's return to the office was no less than chaotic. But, even so, I still have no motivation to get moving. I love the post-run high that I get, and that feel-good mood that lasts and lasts. But still, I can't seem to drag myself off the couch. I adopted this idea from my sister years ago where I place stars (think back to elementary school) on my calendar on days that I work out. I love that sense of accomplishment that I get when I step back and look at the calendar at the end of a month, and see lots of stars. This month so far, the calendar is somewhat bare. Have I mentioned that my birthday is in two days? I wonder if this is causing the mental block?