I ride pretty much the same as you do (with short tours thrown in from time to time), and I have a Marin San Rafael that I bought for about $450 at a LBS. I have been very happy with it. My opinion: you should be able to find something new from a LBS that suits your needs for less than $700, but probably not less than $400. I'd look at 'performance hybrids' or dual-sport models, which are not mountain bikes, but handle the gravel/crushed limestone/dirt paths pretty well.

This is my bike:

http://www.marinbikes.com/us/bikes/d...san-rafael-ds2

The only mods I've made are to add bar-ends, switch out the pedals (still platforms, but the original ones were cheap/flimsy) and add a rear rack. It does everything I ask it to, and is comfortable enough to ride 50 mi in a day. I'm not fast -- but I'm pretty sure that's more about me than the bike :-)