Mendocino County is not a place to explore with wild abandon. I strongly advise sticking to paved roads with road signs and when out hiking, staying on clearly marked paths. There are two main reasons for this:
#1 - Mendocino County is rural and it is easy for even locals to get lost. Your quest to see what's on the other side of the mountain or where that dirt road goes could have you waiting for search & rescue to find you. The search & rescue bill is not a souvenier many are happy to take home.
#2 - This "middle of nowhere" location with easy access to a major highway combined with excellent agricultural land means that there are marijuana gardens and methamphetamine labs to be accidentally stumbled across if one does not stick to the beaten path. Hikers have been shot and killed after wandering off the path to explore and stumbling across an illegal garden.
If you stick to paved roads and clearly marked trails, you'll have a wonderful time without putting yourself in danger.
Don't get me wrong, Ukiah is a great place. But, it is not safe to go wandering around late at night in neighborhoods you know nothing about. There is an active gang population in Ukiah that is often feuding, a proportionally large homeless community (many of whom have mental health needs and are refusing services), and the town is right on Highway 101 (a major road running through the west coast) - which means you really never know who happens to be in town. You probably will not see this side of Ukiah, but It would be wrong of me not to tell you about it.