Hi ell33, I think Ian has summed it up for you quite succintly, your LSR should be that a long slow run 60-90 secs outside your target MP to get your legs used to running for time rather then speed, I know your following a plan but you may be jumping to far too quickly? 1/2 mara to 18/20 miles is a big jump and at above MP will contribute to the fatigue?
I run a 3:13 mara so around 7:20mm, my training pace is around 8:30-7:55mm over 15-18 miles, the quicker pace usually averages from a fast pace final 3-5 miles (not often just when I feel i'm running well) you may be better served slowing down a tad and getting your body used to the longer distance (as you question yourself).
This is your 1st mara and so meeting your target is important but getting to the start line feeling fresh is worth so much more.
My 1st mara was run in 4:22 which I was happy with in hindsight but i'd expected to get around in 4hrs so a touch dissapointed but my legs were just not up to the distance or pace needed, I learned from that and adjusted my training and each marathon brought me closer to my goal, the same will be true for you.
In regards to carbs, a good balanced diet will serve you well and the gels/isotonic drinks will keep you topped up throughout your run/race 'based on the claims
'.
Best of luck with your training and let us know how your training goes, I hope i've been of help and happy running!!!