Congrats on making the decision to do your first marathon. The training runs aren't something to be nervous about, they are just training runs. If you are having a bad day that day you take it slow. And the training runs are so important to being able to make it through the race itself. Just make the best of them and know that someone else on this thread may just be doing a long run the same time you are.
The race is a different story, but I get nervous before every race, even a 10K.
Having said all of that, I am nervous about my run tomorrow. I have plantar fasciitis, and tendonitis in both Achilles, and it is all acting up after my 11 mile long run yesterday. I have 6 miles scheduled tomorrow and it is going to HURT.
The worst part is I really can't afford to take any time off because I have a marathon in October and I am now behind on my training because I had to take it easy for a couple weeks. I had all these plans for my time at Disney and as of right now I am just hoping to be able to walk while I am there.