So, yeah, I would not counsel walking along the départementale (main road) that passes the front of the place. Very dangerous. This said:

1. If, at the front of the driveway, you turn west (toward Senlis) and walk maybe 40 yards (or 36 meters, or whatever), then look across the road (i.e., to the north, up the hill), you will see a gravel farm road that bisects a field. Cross the départementale (carefully!) and walk up that road to the end of the fields where it meets a little country road (rue Georges Dauchy on Google Maps, but I don't remember it labeled). Anyhow, turn to the left (away from Senlis, toward Chantilly) and follow it to the intersection, where you bend right, but then look to your left where you see a bike path (or some such) going into the woods. Follow it! This is the trace of an old rail line connecting Chantilly with Senlis. You are in the woods, but the huge golf course is always to your right and the little towns of St Firmin then Vineuil to your left. At the entrance to the golf course, you can turn left and descend into Chantilly. Earlier doesn't work because the canal blocks you off. All of this can be seen on Google maps -- the astuce (trick) here is the farm road just outside the hotel. FWIW, I did this as a walk with spouse and as a run a couple times a day. Don't know what it will be like in colder, darker weather, but it works now.
2. At the desk, you can get someone -- supposedly -- to let you out the back alley into Avilly (the neighborhood on the south edge of the property) from which it is a fairly short walk to the Chantilly grounds. Good luck getting back onto the property, although they claim if you call someone will come unlock the gate. This is something I think they need to work on.