I set my mile PR (5.59) on the roads after following a dedicated plan that used the treadmill extensively for speed training (hard half mile repeats one day and mile repeats four days later). Treadmill times were similar prior to that then was able to do it on the road.
To me using the treadmill that way can be exciting because it’s hard running that is slightly terrifying and I can set it to significantly harder than I can run on my own.
The most I have done was 11 miles in 2 hours and it was not that bad.