Since I have been a runner, I have really enjoyed making challenging treadmill workouts. Rather than just running for 8 miles straight, I like making sets that have a pattern. This is a short workout I have been doing for a while, but I just recently tailored it to make it perfect. I'm calling it "Death by Treadmill." Kind of a play on "Death by Barbell," a crossfit workout at the gym I go to. It is perfect for those days where you want to get a good workout but can't imagine running for an hour or more. Obviously, every person will be different, but for me, it is at that exact point where I can do it pretty much any day, but I always feel like I'm gonna puke at the end.
The entire workout takes 31 minutes, plus warm down. Here it is: First, run two miles easy warmup. You don't want to do the second half without warming up. The second half starts at 15 mins, so the faster you go, the more rest you get before the second part. I usually do a mile at 8mph (7:30) and one at 8.5-9.5mph (7:02-6:20), giving me around 45 seconds rest on average. After running 2 miles, jump off onto the sides of the treadmill but don't stop it.
During that 45 seconds of rest, set the treadmill at 15 incline and 8.5mph. It is tabata style, so you run for a specified period of time, and jump off onto the sides of the treadmill for a specified period of time. The first run is 30 seconds, and it increases by 1 second every time, so the last run is 45 seconds. 1 second may seem like nothing, but it also decreases your rest, so after 10 mins, you are running 40 seconds and getting 20 seconds rest. You always start the runs on the minute.
After each run, decrease the incline by 1, and increase the mph by .1. So the second run is 31 seconds at 14 incline and 8.6 mph. Then, 32 seconds at 13 incline, 8.7 mph. Adjust the values while you are standing on the sides of the treadmill and before you start again. Continue until the 16th run is 45 seconds at 0 incline and 10 mph.
Personally, I have found the 9-11 incline, 8.9-9.1 mph ones to be the hardest. It gets substantially easier for the last 5 or so. 31 minutes, and I guarantee you will get more out of it than if you ran 8+ miles straight. Obviously, adjust if for your own speed/abilities if you need to, but give it a shot, and let me know what you think.