I took a rest day yesterday.
The thing I liked about the 16 miler we did, is that it was 4 loops of 4 miles. After every 4 miles we are back at the start where you could eat, take a potty break or change clothes. They also had a 50K and a 50 miler. We did one in January that was 11 miles, 33 miles and 55 miles (approximately). The people doing the longer distances brought stuff with them and set up a little camp so they could rest, changes clothes, socks, and eat. There was a time limit but most of the ultras will go on as long as people are moving forward.
There is a 6 hr., 12 hr., or 24 hr. race that was I trying to get my sister to do. It's a 1.1 mile loop that you basically run for either 6, 12 or 24 hours. I'm a bit curious on how much distance I could cover in 6 hours. The only issue is that it's in July so it would be HOT. It doesn't start until 8:30am. I would rather start at 6 or 6:30 as it would be cooler. Also I'm not sure I could run the same distance for 6 hours. Still it would be interesting. I found another one that is a little longer than 1.1, is on a trail and held when the temperatures are a little cooler. We may do that one but it would be next year not this year.