<human>: Can you provide a brief overview of the text?

Hey fellow runners. 

I'm currently training for the Chicago Marathon-my first marathon-and for the most part my training has been going great. However, I've started to develop a really nasty trend I'm hoping I can get some advice on. 

For the past three weekends, I've really struggled on my long runs. I've done up to 15 miles with no trouble, but three weeks ago, after a 13 miler, I came home and got sick, repeatedly for about two hours. I chalked it up to heat, but last weekend I tried for 17, and totally fell apart. Threw up at the 12 and 14 mile marks, had to walk the last two miles, and continued to get sick when I got home. 

On both of those, I was stopping at drinking fountains, and probably taking in too much water at once. 

Today, I struggled through 18 miles-had to take a couple of breaks-but I carried water with me and tried to be smarter about how much I drank at a time. Everything seemed okay, but not long after I got back, I started getting sick, and continued to do so for another two hours. 

I don't eat much before I run, and while I'm going through these spells, I try to just sip water, but can't keep anything down. Does anyone have any advice on how to stop this from happening? It's really crushing my confidence.
<bot>: Recently started getting sick multiple times over the course of a few hours following long runs (13, 17, and 18 miles, respectively)