Immediately upon sharing my post, which was intended to merely provide insight into how one might utilize doubles in order to build a foundation for running ultras, I was accused of being too fixated on mileage and neglecting critical components of ultra training such as terrain + elevation.
I want to be clear. Train for the race that you'll be running. But in my opinion, you don't have to skip a run just because you cannot mimic goal race conditions... for instance, run on technical trails or conquer hill repeats. During the week, it's not feasible to hit the trails.... I just don't have large enough blocks of time or a trail close enough to my doorstep. And I'm sure many are in the same boat....and that's OK. I'm still going to bang out the twelve miles on my plan....And I whole-heartedly believe those miles count....I believe all miles count.....Walking, hiking, running, crawling.....You'll probably be doing a little bit of everything in an ultra.
I would not suggest jumping immediately into doubling 5x's / week..... Nor would I suggest that you should do something just because it works for someone else. There is A LOT of science to back-up how beneficial running doubles can be.
1.) Increases endurance.... I've noticed a HUGE increase in my own..... Running will become so routine that it will start feeling easier and easier. With that, I noticed that I became more efficient (faster pace) even without formal speedwork.
2.)Can be easier on the body.... Because you have a recovery period between runs, there's less wear on the body for the same amount of distance. What makes this even more awesome is the fact that it's been discovered the physiological benefit is the same....Meaning 5 miles twice = 10 miles once.... Your body reaps the same positive training adaptations. Score!
3.)Increased metabolism.....Your body continues to burn calories after your first run AND second run. You will be a lean, mean machine much faster..... Or you just have a larger treat allowance (YES).
4.) You'll be a mental beast..... Doubles can be tough.... It's not all daisies and unicorns. I've skipped second runs in lieu of happy hour more times than I can count. But all those times that I toughed out a run when I just wanted to dive into cheese fries and suck down cheap beer.... those are the runs that really count.... even if you scramble along at a pace that's laughable.... even if "seasoned runners" tell you that the miles are junk.... I'm telling you right now those runs that you hate but endure will help you get through the final miles of an ultra.
Listen, DO YOU. Do what works best for you.
For me, I utilize doubles in order to reach the mileage laid out in my training plan without sacrificing ridiculous amounts of personal time or having to wake up at an ungodly hour. Admittedly, it's rare to touch dirt throughout the week aside from a flat trail that exists 1/2 mile from my office. However, I'm notorious for seeking out brutal road routes with an abundance of hills. Generally, I alternate "higher" mileage days (10+) with lower (< 8). On lower days, I will stick with one run versus two UNLESS I have a yoga class on tap. On the weekends, I am in the mountains every chance that I get. Even if I'm not running, I'm hiking with the family. Again, ALL MILES COUNT. Ideally, I am running one longer run + one recovery run on the weekend. Once Spring starts, races will replace long training runs. You don't have to run 100 miles / week but it's necessary to build a foundation that will enable one to FINISH an ultra..... Only an elite few can squeak out 20 mile / weeks and still run 100 milers. I'm hovering comfortably in the 50 - 60 mile range.... I imagine that will become 70 - 80 with peak training. I'm an advocate for cross-training.... I'm an avid cyclist in the summer so in the winter, I spend significant time on the bike trainer to keep in shape. Additionally, I've been practicing yoga 2 - 4 hours / week. I don't drive so I walk or bike EVERYWHERE.... Thankfully, I live in a rural town where nothing I need is further than a 1.5 mile jaunt away.
And there you have it.
Take what you want. Leave what you don't.
Now for the best piece of advice that I'll ever give you....
Don't forget to pack that cheap beer to celebrate your summits.