I had a slightly scary experience during my attempt at a 20 mile training run for my first marathon. A couple miles into the run I actually experienced what I'm pretty sure was vertigo, followed by actually losing my balance and falling off to the side of the trail I was running on. I have been running for some years now and this was the first time this has happened. I didn't finish my run and pretty much walked most of the way back. I attempted to run very slowly but got a little bit of the sensation again and had to walk. I don't have any medical conditions and I'm in pretty good shape for my age (29). Has anyone ever experienced this? I tried researching online but couldn't find any causes for why this happened. It was about 60 degrees and windy and I was hydrating as I normally do so I can't figure this out. Any suggestions would be greatly appreciated.