What happened to D'Artagnan on the way to Paris in The Three Musketeers?

In The Three Musketeers by Alexandre Dumas, Pere (meaning "the father"), a young man of eighteen called d'Artagnan left his home in Tarbes (in Gascony, in southwestern France) to make his way to Paris, where he intended to join the Musketeers.
The Musketeers, officially named the "Musketeers of the Guard," were the personal household guards of King Louis XIII, who formed the Musketeers in 1622.
Like Polonius in Hamlet, d'Artagnan's father sent him on his away with a wealth of questionable advice to occupy his mind all the way to Paris, along with fifteen crowns, "a Bearn pony, from twelve to fourteen years old, yellow in his hide, without a hair in his tail," and a letter of introduction to Monsieur de Tréville, the captain of the Musketeers, who had once been a neighbor of the d'Artagnans in Gascony.
D'Artagnan's mother, knowing all too well her son's propensity for getting into quarrels, gave him a recipe for an herbal salve that quickly heals scrapes, bruises, and wounds.
On his way to Paris, the young d'Artagnan stopped over in Meung, a market town not far from Orleans, about 150 miles from Paris.
Alighting from his aging yellow horse outside the Jolly Miller Inn, d'Artagnan overheard an aristocratic-looking gentlemen, "well-made and of good carriage, although of rather a stern countenance," ridiculing his horse.
Not one to bear an insult, even to his horse, D'Artagnan challenged the man, and within seconds of advancing on the man, sword in hand, d'Artagnan was set on by two men with sticks and shovels who broke his sword in two and beat him until he lay bleeding on the floor, unconscious.
Unknown to d'Artagnan, the gentleman he challenged was Count de Rochefort, a well-known swordsman, who would later become d'Artagnan's nemesis.
When d'Artagnan had partly recovered from his wounds, he discovered that the letter to Monsieur de Tréville was missing. Even though he now had only half a sword, he threatened everyone in sight with dire consequences if the letter wasn't found, which caused the two men who had beat him previously, as well as the innkeeper and his wife to arm themselves with sticks and broom handles.
After d'Artagnan convinced the people with the sticks and broom handles to help him undertake a search for the letter, the innkeeper came to a sudden realization:

"That letter is not lost!” cried he.
“What!” cried d’Artagnan.
“No, it has been stolen from you.”
“Stolen? By whom?”
“By the gentleman who was here yesterday. He came down into the kitchen, where your doublet was. He remained there some time alone. I would lay a wager he has stolen it.” (The Three Musketeers, chapter 1)

With the mystery of the lost letter solved, at least for the moment,

He then drew two crowns majestically from his purse and gave them to the host, who accompanied him, cap in hand, to the gate, and remounted his yellow horse, which bore him without any further accident to the gate of St. Antoine at Paris . . . . (The Three Musketeers, chapter 1)

D'Artagnan sold his old yellow horse at the gates of the city and "entered Paris on foot."