During this time of year, there are buses that run from Izu-inatori station to the entrance of the highlands and then free shuttles that wisk you right to the top of the mountain from the information kiosk.
Entrance to the highlands at this time of year costs 500 yen per person which includes the free shuttle ride to the top and they offer a few food stalls and guided tours as well.
It can get cool up there, so it's best to take a light jacket.
I recommend taking one of the free shuttles to the top and walking back down to the information center. It takes about 30-40 minutes to walk down.
The Susuki event flyer showing the various walking routes.