Congratulations on your upcoming first marathon. I did mine in October last year following the exact training program and I finished it successfully, even dough at a much slower pace. Since then I have finished another marathon and 3 other half marathons. I think what you are feeling is normal for any first timer. I would humbly recommend that you rest for a couple of days, forgo the short distance running and then try to do your 20 miler, but at a much slower pace. Your long runs does not need to feel like a race, they supposed to be slow and comfortable unless you are racing the marathon, which you are not. Don't be afraid to take walk breaks and make sure you hydrate without, during and after the long run. After my 20 miler I pretty much rested and did the long runs and did just fine for my first marathon.
Now, please consider this with a grain of salt since it is coming from a newbie. There are a lot of people in this forum with years of experience who will give you a better suggestion.
Good luck, please keep us posted. You will do great.
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