Here is Tom Dunigan's excellent page with descriptions and maps of the North Boundary Trail, which is listed as 12 miles, but which you can easily turn into distances up to 20 just by figure-eighting the trails, and hitting some of the side trails. It's a mix of chert and singletrack, and a lot of fun.
Also,
here is Tom's page discussing Black Oak Ridge, which you can connect with North Boundary if you're feeling particularly frisky. Or, masochistic.

It's gravel/dirt, also.
Both highly recommended.
Here's a map to help you get there, and also give you an idea of the location of both of 'em.
I'd say they're certainly safe enough for a single runner. Because they're so long, you'll see the occasional runner or more often some mountain bikers, but you'll go miles and miles without seeing a soul, too.