Get some Answers here:
http://forums.mtbr.com/
fwiw consider a 29er
Xtrng:
Pedal out of the saddle when you ride as much as possbile it is alot closer biomechanically to running.Also do lots of uphill riding.For recovery spin ez gears ez intensity 60 to 80 minutes.
Wheels and Tires:
The ride is very tunable with wheels and tires.My stumpjumper hardtail mtn bike is pretty fast rolling big apple road tires.The same bike rails on the trails with a quick switch to knobbies.
A few limitations of cyclocross bikes as mtn bikes
-limited rear tire width
-high top tube can really hurt your privates
-overall geometry and gearing is more road oriented
-frame is not setup for a front suspension fork.The squishy really helps when descending steep rutted rooty rocky stuff
The 29er uses a cyclocross / road size tire with mtn bike type geometry and gearing.The best of both worlds.
Finding the right bike(s) requires first hand experience.
Bikes lose value quickly a high quality used bike can be a great way to go as well as end of year closeouts.
good luck and enjoy the ride