Last evening I had a discussion with a friend of mind about training for ultras and especially for the timed events and my first 100 planned for February. He strongly suggested I start running doubles and thought that would benefit me more than trying to extend my long runs to 5 or 6 hours or more. Any thoughts or opinions on this? How would you recommend my introducing doubles to my schedule. I'm already running 6 days a week most weeks, but with no set schedule because my days off change weekly which affects when I can run long.

A lot of top long-distance runners use doubles if they can find the time.  Pfitzinger says they are helpful after you get to a weekly mileage of about 70 miles.  Above 70 miles per week it is hard to recover from the single daily workouts and be ready the next day.  I've done them in the past and it may benefit you but I'm not sure it's for everyone.  Are you talking about doing two two hour runs in a day instead of a 4 hour run?  In that case I would do the long run.

Two doubles a week would increase weekly mileage by 10-20 miles.  The average person might not benefit much from increasing weekly mileage from 70 to 85 because you need to recover and avoid injury.

You can start by running two 5 mile runs in a day and GRADUALLY increase to two 8 miles a day.  Just start one double per week.  Increase slowly.  Again, if you are only running 50 miles a week you would probably just benefit from the daily runs of 6-10 miles with the long run added in every week.

FWIW I think running 50-60 miles/week including some real speed work or threshold runs probably would be more useful than 60-70 miles/week of moderate running.  Ultrarunners tend to underestimate the benefit of speed work in their training.  My 2 cents.

I just looked at your log.  You should try to increase you daily runs to 6-10 miles/week and try to get in a faster run at least once a week.  This would benefit you more than doubles.  Keep doing the long runs every 7-10 days.

Currently I'm averaging only 40 mpw with a peak training week of 55. That's one of the reasons why I hadn't considered doubles. At my pedestrian pace of roughly 11:30 - 12:30 for easy and long runs, that's about 8.25-8.5 hours of running with a long run of 3-4.5 hours about every 10-14 days. I take a day off when I feel I need a break. I can see myself running doubles when I work an afternoon shift, e.g. 1-6 pm. Sounds like I would be better off just increasing my daily runs when I work a morning shift of 7-noon or evening shift of 7-midnight.

I double whenever I want more mileage or can catch another group run. But then again, I'm not a top anything, and I'm pretty slipshod about my training schedule in general: I try to get in one or two speed workouts in a week, and try to go long or several times on the weekend... and... that's about it.