What kind of pace are you running, genuinegirly? Have you consulted a pace chart to help yourself warm up more slowly?
Are you working out regularly in the recent humidity? The humidity really took a lot out of me for the first week or two, and I was riding my bike or running in it every day. It influenced me to really cut back on pace for a few days and make sure to try to overhydrate since I sweat so much.
Over 18 miles, you should really be taking in some sort of nutrition during your run. For me, any workout over 90 minutes total will have some nutrition intake - be it sport beans, gu, cookies, or even just gatorade. You can get electrolyte/carb/energy gels to carry on a fuel belt while you run. I am actually looking into the purchase of a fuel belt so I can go on longer runs - past 8 miles, I've already gone through the water bottle I'm able to carry and I usually need to take in some calories to keep going.
Women store glycogen less efficiently than men, so it is VERY important for you to pay attention to nutrition before and during extended exercise.
If you're running a reasonable pace, I'd love to go running with you sometime.
I might need to build up a bit more to 18 miles, but I'm comfortable with a half marathon.