I don't remember much from my physics class, but one phrase sticks in my mind. My professor was babbling on about Einstein's theory of relativity, and the fact that it can be summed up in one statement:
You Can't Feel Speed.
Now, it didn't make me sit up and take notice, and I didn't have a light-bulb moment all of Einstein's theories made sense were suddenly made clear. But I do understand that riding in a car at 60 mph and flying in a plane at 600 mph feels about the same.
I found, however, this has absolutely nothing to do with exercise.
Today I did 10 miles on my bike, transitioned to running stuff, and did a 4.5 mile run. And I can honestly say that I felt like I was barely crawling along when I was running, even though it was just a tad slower than my 10K pace, and about a minute and a half faster than my target pace.
I tried to slow myself down. I failed. I tried to feel like I was going fast. Failed at that, too. So I just ran along at my "feeling" slow but "relatively" fast training run. And now I know something about doing triathlons... I'll never feel as fast running as I do riding a bike. If I were to try to do so, I'd probably be quite SOL. :P