A Hardtail with LX groupset as a minimum-Discs are not necessary but are nice providing they are a known make that works- Rebuildable fork- and preferably wheels that will take the punishment you are going to give it.

Or for your trails- a 29er with basically the same as above.

Sizing and you will probably need a smaller frame in comparison to a road bike- but that top tube will be looooong

I ride a 51 road bike and a 15" MTB. Direct comparison on sizing is not possible between road and MTB but the 15" mtb is the comfort bike for long rides. I also ride a 19" Kona that is faster- more agile and being steel is smoother-but I can't do the milage on it. 40 miles XC is enough on the Kona.