Trail running is an ultralight sport. Most who partake have 20 oz. of water in their right hand and 20 oz. of water on their hip...this is my experience on trails that see a lot of use by trail runners. That's it!
I'll try again, going without gear is not "ultralight". It's called "unprepared." The term "Ultralight" is a lighter version of something that works just as well as the heavier version. It typical means a base weight in the 5-12 lbs range, not ZERO. Just saying that ultralight backpacking gear would be a good survival kit for ultramarathon runners on long training runs without the support of a race organization.
Ultramarathon runners typically wear a vest with room for the gear I described above. Some runners you see on the trail might even be carrying a few survival items stored out of view. Here's a typical vest with 12 liter capacity and it comes with a "survival blanket" and a whistle. It's clear from looking at the gear outlets online they are thinking about survival gear. They could learn a lot by crossing over with ultralight backpacker gear.
