I don't run, but I ride reasonably long distances. I have sometimes got my basal reduction just wrong enough that I don't need to/can't eat at all while riding (I don't want to take bolus if it can be avoided unless I actually stop and have a decent sized meal/snack). Riding without eating anything becomes quite hard work after say 3h, but I guess it's no worse after 5 or 6h, which I think is the longest that any of these slightly misjudged rides took.
It's much easier if you do eat some carbs (as it it's easier to produce power at a lower perceived rate of exertion, and you can also sustain power for much longer, I assume similar will hold true for running) - I aim to adjust my bolus so I need on the order of 40 - 60g of carbs an hour (so I have a calorie deficit). Perhaps I could go faster if I were consuming 100g+ of carbs an hour, but I think I'd be feeling pretty sick, so probably not!
While my top end power duration also takes a dive after a long ride (~200km), I do still feel better then than at the end of a shorter fasted ride from the point of view of perceived effort.
I've been riding to work fasted recently (due to FotF) and while it's only an hour, it does feel so much harder than riding when I've had a bowl of granola.