Ask yourself if you're definitely going to take it on the trails, and how regularly... Most road bikes and hybrids are shit for anything decent. Cyclocross bikes will stand up to it of course, but whether you'd want to train regularly on one is another matter... I've taken my road bike off a few times (on-one pompino, so steel and kind of borderline cyclocross, fixed/singlespeed though) and it felt pretty good, but never done it for more than 20 minutes or so and not on anything difficult or muddy. Don't know if there are any cyclocross riders on here who know more.
MTBs can be ok for road use, it does kind of depend on the hills, your fitness and how much effort you're willing to put into. Ideally if you take that route you want forks you can lock out (a lot of them do this now) and road tires. Problem with road tires is they're shit for off-road and you don't want to be changing the tires all the time. Other than it depends on the bike of course, an XC whippet will carry you up most hills nicely, but the geometry isn't so good if you want to try any jumps/drops... Basically road bikes are shit for off-roading, but MTBs can be OK for roads - commuted in London for years on one (on an on-one in fact) with no problems except that i cycled off road a lot less because of not being arsed with the tire changing.