Getting hard while exercising

A lot of folks have posted hot videos of themselves fully erect while jogging or cycling. I have to ask: how did you learn to do that? Have you always been able to do it? For me, alternating-leg motion is an instant erection killer. I can be fully hard, walk 50 steps while thinking about something sexy, and be soft. (This can be handy if you're in a semi-public place and hear someone coming but have 30 seconds warning before they catch you.) I could ride all day on the back of a bicycle built for two with the most beautiful ass I've ever seen on the seat in front of me, and never feel a twitch. It's quite frustrating, really; if I go for a hike in the woods with a friend, they might be ready to go the instant we stop walking, while I need to rest and begin from scratch the process of getting hard to play. I kind of assumed that this was a biological fact, like the way lizards cannot run and breathe at the same time. But apparently it isn't. Anybody have insights?