It's not immediately obvious that Bardelys makes it to Lavedan the same day. But he is in Grenade, which is between Lavedan and Toulouse (on the way back from Lavedan to Toulouse "it had been my purpose to change horses at Grenade", Chapter XX) and from Lavedan to Toulouse is only 60 miles ("twenty leagues", Chapter XVIII). Since he is in a hurry, he undoubtedly arrives midafternoon of the same day, extracts the promise from Roxalanne, and returns as far as Blagnac.
This reasoning is further supported by the fact that Roxalanne is reminded in Chapter XIX of a promise she made "three nights ago" to Chatellerault. Also, Bardelys encounters Rodenard "who I had sorely beaten four nights ago", which is correct since it's by then very early morning.
(There are a few obvioulys misstatements here. Bardelys says that Saint-Eustache learned his identity "two nights ago" at Toulouse [Chapter XVIII]; in fact it was three nights ago and certainly couldn't be less. Similarly, Roxalanne lied to him "three nights ago" [Chapter XIX], but this must be at least four since she saw him Saturday. More surprisingly, Bardelys thinks that his arrival in Lavedan occurs during "the first week in October" [Chapter XIX] which is simply impossible.)
Some praise Bardelys for his mercy in forgiving and reinstating Rodenard. But it is not so; in fact, Bardelys continues to treat his servant cruelly, mocking the severity of his wounds: He promises Rodenard "twenty golden Louis to buy unguents for your poor shoulders" [Chapter XX]. Since the Louis d'Or was first struck in 1640, eight years later, Bardelys is actually saying "You'll wait a damn long time before I pony up any cash, and even then you'll still be in pain." Nice guy, eh?