You can get a lot closer than 8 miles. The Lassen parking lot is only 3.1 miles from Sulphur Works, and 4 miles from the south entrance (the furthest you'd have to walk).
Just follow the road to Sulphur Works, then shortcut northeast of Diamond Peak through the woods to where the road heads north at 8600 feet. Another 100 foot shortcut below the switchbacks at Pilot Pinnacle and you are there in just 4 miles.
It only takes a couple hours and for anyone in OK shape Lassen is still a reasonable dayhike.