Some days are just harder than others

Did you ever notice that you can run the same distance in the same time on one day as another but that one day you feel great, like you can run all day, and the other day every stride is a struggle?  Why is that?  Diet?  Sleep?  It's probably a combination of a lot of factors but it's a tough one to figure out.

On Black Friday I had nice 50 degree weather and the sun was out.  I went to the Metro Parks for my long run of the week.  Within the first 2 miles there are some really big hills.  One is kind of gradual and the other is straight up.  Maybe running these hills so early in my long run is why I felt this run was so arduous.  However, I've run other long runs where I start with hills and then I feel really good after the hills because they're done and everything feels downhill.  Hmmm?  Don't know the answer.  I still finished in a good time for me; about 8.5 minutes/mile for the 12.  Not a bad time, but I felt pretty beaten up afterward.  I'm going to run long this coming Sunday.  The weather's supposed to be great and I'm really looking forward to getting outside without all the cold weather gear of this past week.  We'll see how the run goes.  I'm excited!  I think I'm doing a half.

Here is my past 2 weeks of training runs:

2 Mondays ago - 5 in 38:29

2 Tuesdays ago - 5 in 39:06

2 Wednesdays ago - 5 in 40

Black Friday - 12 in 1:42ish

Sunday - 5 in 38:57

Monday - 5 in 39:12

Tuesday - 5 in 41:50 (??? don't know what happened here ;-) although I did run a big hill on this day... Foster's Run... anyone run it?

Thursday - 5 in 39:19

Friday - today will be 5 in ... I'm guessing here... 39:32

Here's another question for the masses out there running around; when you run do you always plan your run?  I always know exactly how far I'm going and I don't deviate from the plan.  Sometimes this is good and sometimes not.  But this is a function of my personality.  What if I have extra energy?  What if I have none?  Should we deviate from our plans?