I went for a 5k run around 8p this evening. The boring route: just along Middlefield Road from my place to San Antonio, back to Shoreline, then back to my place. Total time was 26 minutes but included 30 seconds waiting for a traffic light and 60 seconds taking a drink while walking 100 yards. It was about 70F outside. I wore my CamelBak, which is arguably overkill for a 5k urban run but was a pleasant companion by around the second mile point. Unless I've been hydrating for a few hours before running I'm nearly always dehydrated within 15 or 20 minutes of the start of a run on a warm day (anything hotter than 56F is warm for this purpose).