Or, if straight bars, get some bar ends and like with drop bars, change your hand position often. I'm firmly convinced that bar ends are necessary on flat bars. I've ridden a lot of miles on a flat bar hybrid, and I always felt it in my shoulders & upper back. When I got bar ends, that pain went away.

Nerve impingement. You don’t provide enough information to suggest a specific solution but in general if you move your hands more it’ll happen less if you hold a fixed grip for long it’ll happen more. Straight mtn bike bars put your hands in an abominable position for riding in a straight line. So if you have straight bars consider extensions of some sort or conventional swept back “touring” bars If you have drops move your hands around more often. Of course gloves and grips help. If your arms are locked bend them and flatten your lower back a little. Cycling for efficiency/fitness think of suspending your upper body over the bars with your upper body dynamic and your hand grip floating occasionally.
Putting dead weight on the saddle or your hands isn’t good. Make your weight live. Tiny movements can make a huge difference.

Straight bars was a straight copy of motorcycles with no consideration for cycling. They’re good for descending but that’s it.

Take it from me, riding 50 miles with straight bars is rough. Only one hand position, upright, and as I said I felt it in my shoulders and upper back.

Thanks for all the feedback. 24 hour hold is over. Guess I have to be very selective on what/when I post.

Definitely going to monitor my positioning on my MTB. Yes, I have straight bars but will look into getting an add-on or completely different bars since the bike is primarily going to be used on a paved trail that just opened up near my home. It used to be old railroad tracks and now is a great trail through the middle of town. Not sure how long it goes but probably 20+ miles from end to end. Just need to stop at intersections which gets a bit tedious. I'm 53 years old and in decent shape but have had three lumbar spine surgeries in the last 7 years. Need to ease back into biking. If my body does not fail, I will most likely buy a road bike. There is a Play-Tri store in my town that I have yet to visit. Cervelo and BMC have models that seem to catch me eye at the moment.

Since you mention you are using flat bars, try something like ergon grips with the integrated little bar ends.

In my experience, these can be just as good on long rides as drops.

5 miles is early; if you'd said a couple hours then we'd be taking about tweaking your fit and moving hands from time to time. But 5 minutes, that's a different matter; something's wrong from the start.

Somebody mentioned sliding forward on the saddle; if that's happening, fix it with saddle fore/aft, saddle tilt, or with reach.

Wrist angle is a thing. If not neutral, fix it.

It there's too much weight on your hands, fix that. Again saddle fore/aft, bar drop, reach.

So you have the bars in a death grip? Fix that.

If all else fails, pay a fitting. Or just pay for a fitting from the get go.

Yeah, I learned to relax in the saddle. The Ergon grips both flattened out my wrists and provided the bar ends for alt positions. Now it is not unusual to ride all day with no issues.