I've noticed this too back when I ran a lot. The trick I use when this starts to happen is to tense my thigh and abdominal muscles. I believe that the legs have veins designed to return blood to your core (and hopefully reach your head) when muscle contractions squeeze the veins. It seems to work for me (almost instantly).
I have that problem, and I don't exercise at all. Happens probably once or twice a week. Almost always from going from sitting to walking too quickly.
Usually, as long as I grab onto something and stay still, it passes after about 20-30 seconds of my vision blacking out and my thoughts getting all jumbled and incomprehensible.
I've only fully blacked out from it once, coming to and realizing I was twitching on the floor.
I've been led to believe that it's a fairly common condition, though.
Interesting. I've been a runner for over a decade and run 50+ miles/week. I experience light-headedness going from sitting to standing a few times a week.
I bicycle 150 miles a week mostly on steep mountain roads. I seldom faint, but hopping out of bed to go take care of chores in the morning usually results in brief loss of balance and eyesight as my head loses blood pressure for a few seconds.
Sometimes I have to sit down in the bedroom door frame so that I don't faint and collapse. Once I'm up though, my body is full of vigor.
The article points out that everyone will experience this at some point due to extended exertion. This girl's condition makes it happen more quickly than average.