I did 9 miles in an hour once, at which point the treadmill went into "cool down" mode automatically. Didn't have interest in running more anyway as the mental strain was definitely starting to get me. I find that on a treadmill/elliptical, the first 30 minutes is easy mentally, but by an hour I'm definitely ready to be done doing cardio in place
When I was in grad school I worked in a fitness center and often would run on the treadmill (Woodways) when my shift was over.
It was best to just do 10 miles at 10.00 mph. I didn't want to spend extra time "at work" so that was the fastest way to get it over with.
In the last decade though, the longest I've run on a a treadmill is 6M. There is no real reason not to run outside. The weather is always runable here in the PNW.
I ran a marathon on the first day of a vacation. I couldn’t run at all the previous day due to travel, so I decided to just double up on mileage for the day. The treadmill had a 1 hour limit, so I had to restart it. Ran sub 3 for the running portion. Never again.
I used to do a ton of long runs on the treadmill. I have done 24 a few times, 22, 20 about a dozen times. I actually think it’s not all that bad and helps you lock into a pace and just forget about stuff for awhile on those longer runs.