He sped up to match her incredibly fast pace, but had no problems keeping up with her. Emmett was used to not doing any work when it came to hunting; his senses did it all for him. He waited until they were a good ways into the woods before letting them take over, since he wanted to be a safe distance away from humans. When he caught the scent of something rather peculiar, but appealing, he followed it, knowing Rosalie would do the same.