I utilize a long run every other weekend when training past the marathon distance but usually only go to about half the distance in one day, adding more on other days. For example when training for the 100 miles did 2 hours Fri, 8 Sat, 5 Sun which totals 15 hours of running but the big workout of 8 hours equals 80k, half the distance of my race.
This is a good article, I don't know if this will answer your question, let me know what you think.
http://www.pponline.co.uk/encyc/0869.htmJust remember "If you undertrain you might not make the finish but if you overtrain you might not make the start."