<human>: In 8 days I will attempt my first marathon.  I have two half marathons under my belt this year(2:05 and 2:15 were my times) after starting training at the end of last year.  Since June, I've been preparing for this upcoming marathon.  

However, I have been struggling with my long runs above 18 miles. A month ago, I ran 19.23 miles (but walked the last mile after cramping) at a 11:37 pace and this past weekend I could not run more than 16 miles without cramping up but at a 11 min pace.  When I cramp up, my legs get so sore all I can do is walk.  Every other weekend I was able to run my goal so long as it was in the 8-16 mile range.  I admit to not following my nutritional goals as well as I should so I think I can do better on that front.

Can you condense the text into a few key points?
<bot>: I am struggling with runs about 18 miles that force me to walk.  Do you have advice for a first time marathoner who is disappointed in his training runs?   Thanks.