3.5 miles on a treadmill. Interestingly, I find it a lot harder on the treadmill vs. outdoors. I do a similar speed. I wonder if this is a) due to relative boredom, or b) my iPhone GPS is not working correctly. E.g., my typical run is 5 miles outdoors. I feel it, but it's not a terrible stretch for me. 3.5 miles on the treadmill and near the end I was grabbing the bars every once in a while, sweating like a pig, and feeling a little off. Maybe a one off thing.