Something that you don't specify is for how long are you going to be riding over roads. If you are going to ride exclusively over roads get a road bike, even more if you are going to ride over long distances. There is nothing better, period! Also, Are you riding alone or in a group? Don't spect to keep the pace riding with a bunch of roadies if you are on a mountain bike.
Now if you are like me that ride up to 30 miles of a mixture of roads and rough terrain (from mud to soft sand) all in the same day you should consider other options. Cyclocross bikes are a good match for what you are describing; it all depends on how technical the trail is. You can also take a look to a 29er mountain bike. 29ers have standard mountain bike geometry frames that accepts 700 wheels with either slick/narrow 700x22 tires or the so call big/fat/knobby 29x2.1 inch tire (which is a 700x52 tire), or you can go like me 700x42 IRC Mythos slick (which is slick only in name) or the 700x44 WTB. Gary Fisher make a couple of models, mine is the Dualsport 129. Something else that I like in this model is that the chainrings are bigger 48/38/22 instead of a 42/32/22 like in the Tassajara; the rear cassette is a standard 9 speed 11-34. I was a little concern about this when ridding on difficult terrain but I get used to it and I'm happy. Also the fork has a lockout system so you can "turn" off the suspension when riding over roads. There are even full suspension 29ers but since you are going to ride over roads I think that a hard tail will be more efficient. Take note than even with all those changes a dedicated road bike will be always better over roads. Other style to consider is a hybrid bike, but take care when talking about them; members of these forums are a little hostile against hybrid bikes.
Now, the Tassajara is a great bike if I'm you I will spent much less money and make some of the changes suggested by other members. Try more narrower/slick tires first, it doesn't have to be a traction-less tire just less aggressive. Also try changing to bigger chainrings as suggested.
Good luck!