Tristram estimates that it's taken you about an hour and a half to read as much as he's written since he wrote about Obadiah heading off to fetch the doctor. (Anyone else check their watches?) So he figures enough time has passed for Obadiah to return.
In the meantime, Toby has come all the way from Namur to England, has recovered from a nearly mortal wound, and traveled 200 miles down to Yorkshire. Whew.
Luckily for any critics who might have a problem with the way that Tristram is handling time, Obadiah meets Dr. Slop not 60 yards from the stable door.