Posted: 24/03/2014 at 15:57
You had a rubbish run and you know why. It happens to all of us and there's nothing we can do about it. Forget about it and remember the good runs you've had.
As to how far you should go for your next long run, do whatever your schedule says. You're in taper, overdoing it now has the potential of leaving you too tired to do your best on THE DAY.
... I get very nervous /overwhelmed when the distance is over 17m ....
This may be your biggest problem so maybe you need to think of some positive strategies to get over it. You said you had a good 18 miler so focus on that or maybe break the race down into 10k chunks. You know you can run 6 miles, you just have to do it four times in a row