Just this last week I put new tires - Kenda Kross Plus - new cassette, and new chain on my Trek 820. It really needed them. Last night I took a 12 mile ride around the 'hood to make sure everything was working properly.
Today, I took a 38 mile ride on the Katy Trail which is flat and topped with crushed stone. I really noticed how much more uncomfortable this bike is for these kinds of rides than my Raleigh hybrid. The Trek has me leaning forward with much more of my weight on the handlebars, and after a couple of hours it got uncomfortable. I can easily do 50+ miles on the Raleigh and feel fine. I usually use the Trek for 10-15 mile rides around home on gravel and some pavement.
Maybe it was the heat, but I don't think I will repeat that kind of a ride on the Trek soon.

So, make it fit. It wasn't made for what you are using it for. But it can be reconfigured with a few adjustments and parts swaps.

I have a Trek 820 that I use for an "about the town" bike. I put an adjustable stem and a riser bar on it to bring the handlebar positions up about 3" from stock. Much more comfy now.

Too much weight on hands generally means bar too low or saddle/bar reach too long. MTB's are not different from any other bike, and need to be fit to the rider and the task. MTB's also use bar ends to provide alternate hand positions in addition to leverage for climbing.

Had an 820 many years ago and for offroad it was not that good. As a town bike or gentle trails it was fine. So if it did not cut it after only 38 miles- time to get sell it and get a real offroad bike.

https://www.whytebikes.com/whyte-2007/index.html

One inexpensive solution might be to get a stem riser. I bought one from Performance Bicycle for my Balance MTB a decade ago when my then aerobelly interfered with my breathing if I hunched too far over the handlebars. I've lost the aerobelly but still have the MTB.

By coincidence I was riding my recumbent trike through my neighborhood a few weeks ago when I came across a silver-gray colored Trek 820 propped up against a garbage can along with two cheap kid's bikes. I grabbed the Trek and locked it to a nearby light pole until I could return home and get my car. I spent an hour or two cleaning, lubricating, and adjusting things on it. Everything worked on it and it only needs new tires. It's now in the hands of a kid who had badly outgrown his own bike and could really use a bigger one.

Stem riser, huh? Sounds like something to look into.

I don't think the stem on the 820 can be raised, can it?

Stem risers are $15-$20, raise the stem by 2.5" - 3"

Likewise a 3" or 4" riser handlebar will also cost $15-$20.

Just cross your fingers and hope your cables are long enough.