Maybe lean your torso forward more, bend at the hips. Depends on the bike fit, the handlebar location. Put yourself and the bike in front of a mirror and you can see what is going on.

But note that on flat land, I don't think there is much need to pedal standing for more than a "few hundred meters". Usually you stand to ease over a patch of rough road, to stretch your legs, to accelerate, to sprint, and those don't last long. Where some people pedal standing for longer stretches is when climbing. Even climbing a hill, for me anyway, it would be unusual to pedal standing for over a mile or so; others may do differently.