Last week I did a "spring conditioning" 17.5 mile day-hike with 2,000 feet gain in 5 hours and 40 minutes. Went home and walked the dog another 2 miles. Had I been able to keep up that pace for a total of 15 hours (not likely at my age!), I could have met the challenge. 50 miles at 3 mph (a reasonable day-hike pace) is about 17 hours. That is not too outrageous for a fit 20-something if there were minimal elevation gains. However, a bit difficult to impossible for those of us 70 years or older.