I don't think they could. I watched a few videos on youtube by Gary Hughes recently, who had several videos of footage of the exact spot on the bridge and figured out where the girls were - if you look at the photos of the suspect on the bridge, he is starting to walk towards their direction, and that portion of the bridge places them not at the end of the bridge, but cornered on one of the overhanging platforms - they were not at the end of the bridge. I think Abby and Libby were hoping he would just pass by them, instead of attempting to outrun him, because at this point they didn't have any real legitimate urgency as far as they knew, he was just a weirdo to them. And last but not least, you cannot run on this bridge for the life of you even if you tried, you have to look down the entire time you're walking, because of the gaps in the wood, or else you trip and lose your balance. I think before and after that snapchat footage was snapped, he started talking to them, or did something to exhibit odd behavior (which prompted the snapchat footage), and they both hoped he was just a weirdo that would pass on by them, and that would be it.
So, my theory is that the suspect approaches the girls, they're cornered because the only way off the bridge at that point is to jump off the platform and for sure die or seriously injure themselves by falling below, or follow his instructions. I think he threatened them with a weapon to intimidate them and told them to start walking, Libby has the presence of mind to turn on her camera to pick up audio, most likely concealing it in one of her jean pockets, all the while both Libby and Abby think they have the potential to run away once they get off the bridge, but something goes awry once they do make their way off and attempt to escape.