I'm 50 years old, have been riding about 500 miles per year. This year I plan to ride more...have about 500 miles in 2 months. Typically have been taking 10-15 miles rides with a longer ride one time per week. Yesterday I tried my first 50 miler. About at the 35 mile mark I experienced either cramping or muscle fatigue in front of my upper leg muscles...especially the right leg. Does anyone have an opinion whether this was cramping or muscle fatigue (which I never experience on shorter rides). I had taken in water/electrolytes/carbs prior to riding and about 40 oz. of fluid during the ride. Any thoughts would be appreciated.

You pushed your legs too fast to soon.
Took me 1000 miles to get my legs ready for 50 mile rides.

I take natural Magnesium to prevent cramps.

+1

When you are increasing your distance increase it no more than 10 or 20 percent at a time.

You'd know it if it was cramping ... cramping is really painful. Muscle fatigue is just a feeling of tiredness in the muscles.

Drink lots, and don't forget to eat regularly.

If you're aiming to do rides longer than 50 miles, you might want to start posting in the Long Distance forum.
https://www.bikeforums.net/long-distance-competition-ultracycling-randonneuring-endurance-cycling/

since you aren't very conditioned maybe you should have short pauses in your long rides to stretch your body.