For years my only bike was a mountain bike and I would put in the occasional day-long ride. My suggestions echo some of the previous ones: Get some higher gearing, which you may be able to accomplish by adding a larger chainring up front, although the design of many MTB frames may limit how big. At least get some bar ends to help vary hand positions. I have extra-wide handlebars thrown forward with inboard mounted bar ends and an aero bar, which really helps on the speed and helps vary hand and seating positions from upright to prone with a couple of stops in between, as the elbow rests can also be used as hand grips for very upright rising. I'm in my mid 50s and have had "hand issues" for a few years and pad my bars with foam pipe insulation. And the biggest suggestion would be smoother, more road-oriented tires, assuming you are riding mostly pavement. The hybrid road/offroad tires with a smooth center section but knobby "shoulders" are a possible compromise. They roll well in a straight line, but corner poorly due to the knobs on the shoulders. When run at lower pressures they do slightly better in the dirt and mud than smooth tires, but not as well as dedicated knobbies. As far as the saddle and padding, that is a very personal matter. I prefer and have had luck with gel seats, and a seats with springs, although for some people this leads to chafing. Some people have more comfort with hard saddles...not me.

I have no experience with front shocks.

There are conventional approaches to everything, but that doesn't mean you can't deviate or try someting different. For the record, I now have a semi-touring, drop bar bike I commute on, and that is what I would take for a long ride. I haven't taken it off road, but I did fit my old steel road bike with 32s and have had that off road in the foot hills. It's not optimal, and I houldn't jump on it, but it worked.