26 year old in good health could walk out today and do 50 miles if you gave them long enough. It wouldn't be pleasant however.
With 14 weeks, its not ambitious at all to do 50 miles, assuming that he's willing to ride some between now and then. Ride 4 times a week. First week 5 miles Tuesday and Thursday, Saturday, with a 10 mile ride on Sunday.
Repeat, adding a couple of miles to each ride each week. Within 2 months, he should easily be doing 25-30 miles for his long ride. From there doing 50 is not a stretch.
After the first two months, add some intensity to 1 or 2 of the workouts, either actual intervals, or just sprinting for city limit signs, or ridining one block easy, one block hard, that sort of thing.
This isn't rocket surgery.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.