My opinion, I'd stay with a mountain bike. You can swithch out the tires. Knobbie tires for dirt and gravel which are wider with tehm little knobs on them made for trction in the dirt.

I fyou feel like a littel more speed, switchthe tires to slick tires, more narrow and higher pressure for more speed and less rolling resistance.

SOme might say go with a hybrid which is half road bike and half moutnain bike. But many opinons say that the hybrid doesn NOT do any one thing well. I'd say go with the mtb and a set of each tire type.

Now if you want to do long distance 50-100 milers (which most riders end up doing), go with the skinny tire road bike.