Kings Mountain - Skyline Boulevard - Canada Road Loop is a 22.3 mile moderately trafficked loop trail located near Redwood City, CA that features beautiful wild flowers and is rated as difficult. The trail offers a number of activity options and is accessible year-round.
Great hike in nature; but beware that it's downhill one way and then uphill the other way back if you do the first loop ... it can be pretty steep at times but well worth it if you want to be surrounded by pure nature.
I really like this route. It's has very very few cars, quiet, serene and quite smooth. Yes, it's all uphill which requires some normal level of fitness. The bends are awesome going downhill.