The sneaky Man in the Yellow Suit has convinced the constable that he wants to get to the Tucks's house in order to save poor little Winnie from their evil clutches. It's all a complete lie, of course; old Yellowbelly just wants to get his greedy mitts on the forest and with it the spring of eternal life. However, he can be quite charming and plausible when he wants to be, as most psychopaths are, so the constable is completely taken in by his phony story.
Even so, the Man in the Yellow Suit is so impatient to get his hands on the Tucks's forest that he can't wait for the constable. He suggests that he should go on ahead, as he can get there much more quickly on account of the constable's being an older man. Once he reaches the forest, he says that he'll keep watch until the constable arrives. Yet again, the Man in the Yellow Suit is trying to make it seem like he's the good guy, doing a favor for someone out of the goodness of his heart, but yet again, he's lying through his teeth.