Believe it or not I read yet another 1981 article from Runner’s World entitled “The Easy Way to a 50-Miler” by Tom Osler. He says you should be able to run a 50-miler off of marathon training by following 3 simple techniques:
1) Developed a relaxed, playful, non-competitive attitude toward the effort.
2) Mix frequent short walks with the running.
3) Drink ample quantities of sweet drink.
For #1 he gives a chart that includes best marathon race times, best 50-mile race time, recommended 50-mile race time and 50-mile training time. For example, a 3:15, 8:00, 8:20, 10:30. So if you’re a 3:15 marathoner, you should shoot for 8:20, not 8:00 and your training pace should be what you need to run a 10:30 50-miler. You probably need to adjust this based on the course, conditions, etc.
For #2 he suggest different walking invervals based on your marathon time. For example, a sub-2:40 marathoner should be able to race 50 miles, without any walking. A 2:50 marathoner can run for 20 minutes, walk for 5 minutes. A 3:15 marathoner should be able to run for 10 minutes and walk for 5 minutes. As with everything, experiment in training to find the run-walk ratio that works for you.
#3 may have been written before sports drinks as the author seemed shocked to discover that sugary drinks could help him run longer.
He does mention long training runs. The goal of these runs is to cover 30+ miles without generating significant fatigue. He even says you should be so fresh that you could easily do a regular workout the following day.