Marathon training

I feel like I'm standing at the precipice.  To marathon or not? I'm 8 weeks out and based on my run this morning I think I really should throw in the towel on the 26 mile idea and stick with halfs and shorter.  BUT, the marathon I have been training for is perfect for me.  It's small, cheap and on the route that I run regularly.  There will not be a better opportunity to give a marathon a try than this one(ok there's always next year).  I'm taking a slightly different stand to my training for the marathon than I did for the 30k.  With this plan I'm running 4-5  days a week with 1 long run(10-20miles) 1 med run (5-7 miles) and the other running days are speed/interval work.  In the past I have just put as many miles under my treads as I possibly could.  This time I'm spending my energy differently. I am maintaining my high intenstity cross training sessions and the interval/speed work is new for me.  My hope with this plan has been self preserverence.  I was not in my best form during the 30k and I don't want that again.  BUT, here I am today having cut my long run short (really short, from 13 down to 5) just because I wasn't "feeling it".  Does this mean I don't have it in me for the a marathon?  I'm not sure.  I realize that mental preparedness is nearly as important as physical when it comes to endurance running but I'm not sure just how to effectively train this aspect.  Ideas anyone?

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