In reply to rossowen:
Sounds like a combination of factors. Primarily upping your mileage considerably. If you are used to running 5 miles, then putting in a 15miler is a big step up and likelyto leave you dehydrated and lacking in carbs.
I would also suspect your 5 miler the day before depleted your carb levels more than you think. Try to have something like a chocolate milkshake after a run - triathletes swear by them, or Rego or similar post training drink. keep an eye on your hydration and salt levelstoo. Try to flush out the lacticacid with lots (butnot too much) water and put a Nuun tablret in to prevent you losing electrolytes during the run and in the hydration period thereafter.
My ex is on a tri training camp in Spain at present and rang me in some distress on Monday after she blew up on a hilly 3 hourbike ride. She had neglected to fuel properly while travelling out there and her fast metabaloism meant she was perilously low on energy when she set off.
To condition your body, try one fasted early morning run a week, but make sure you carb up on return, through the day.
Also avoid anything too sugarey, including fruit sugars. This can lead to sugar spikes, bad moods, poor performance and interfere with your blood sugar levels.
Basically, you bit off a bit more than you can chew at this time and knackeredyourself. i have done it myself. A bitof food and rest and you will be ok.