Feeling Faint and Frustrated

Well, I have been running the last two evenings at Lake Merritt. Both times I had to walk a bit because I felt a bit faint toward the end of the run. I don't really know why. It isn't like I haven't been eating. I even took a Gu before going out tonight. I think I lasted longer before stopping to walk than last night, but still didn't make it all the way around without stopping. I am wondering if getting my period has anything to do with it. Also, I am worried. If I can't even make it 3 miles, how the hell am I going to do 13?!? Yikes. Well, my period should be over by then. I plan to do a longish run over the weekend. So, I will have to see how that goes. I plan to take water and at least one extra Gu with me. If anyone has any advice or comments, I would love to hear what you know, think, etc. Thanks.