Coming from Montreal this was a perfect little 1 day roadtrip/hike, took the back roads to get there (Saint-Chrysostome, same time as highway) went through a small borders office (that closes a 4pm)
there's a nice park at the end of Chazy lake, keep an eye out for Lowenburg road to get to the Lyon mountain parking lot, the sign is hidden behind trees.
the hike was great! Starts ordinary and gets more and more technical. Crossed a couple of small creeks, lot's of fallen trees, moss, big boulders etc...
took us 2h to get to the top and 1h15 to get back down, it was a 13km (not 10km as this page says)
The top is amazing, the view is incredible!
That shaky/rattling fire tower is a bit sketchy at first but well worth it, watch your head when you get to the top...