A few thoughts:

First, I'm not a physiologist so cannot explain particular physiological reasons why increased mileage leads to greater running-specific adaptation, but as a reasonably experienced triathlete whose been limited at times, and unlimited at times due to various running injuries, here's what I've observed:

-Increased weekly mileage does tend to incrementally improve average speed. Physiologically, I find this manifests itself as the number of miles through the run at a steady pace where I start to "notice" that the running has taken a toll. For instance when I'm doing 25 MPW that may end up being mile 10-11 in a half marathon. Whereas if i've been doing 15, it's more likely going to be mile 6-7.

-As another proof point, check out fivethirtyeight.com's marathon predictor (https://www.google.com/...=chrome&ie=UTF-8). If you input an arbitrary time for a given distance, then change only the slider controlling average miles run per week, you'll notice that more miles = faster times. I believe this calculator is based off of an aggregation of survey data from runners. Thus, I think it is more reliable for the idea that higher mileage = faster times.

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CG

20 mpw isnt much time, even if you're running 9 or 10' miles. So the big reason more increasing miles helps is because you're barely doing any of it to start. The nuances of muscle recruitment, concentric vs eccentric loading, etc matter as well, but are initially swallowed up by 'run more'. Generally you can't replace mileage with other conditioning up to a point. What that point is depends on the athlete and training being done. Could be 30 mpw, could be 60 mpw.

I entered a 50mi ultra train run last year after not much run training. I did ok for about 35mi, then started to really slow down in the race. I got lost in the dark with little food or water and the light battery running dim and had to DNF in the cold at 47.6mi - so close......

I don't think you need to focus on having a specifically heavy run training period, so long as you focus on having your cardiovascular system being used to being active for 5-6hrs of effort.

If you're comfortable for being active for these long periods of time, a 50mi Ultra should be attainable.

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https://www.strava.com/...tes/zachary_mckinney

How much of a running base did you have? Both in terms of years running and previous mileages?

It's much so easier to get through a 50 miler "relatively comfortably" on limited mileage if you have previously had significant miles in the bank, even if it's going back a few years. In those situations, prior race experience counts for a hell of a lot too, in relation to whole manner of things........pacing, race strategy, clothing, equipment, nutrition, navigation, general ultra running nous.

Why are you getting injured so often?

Consistent mileage is king. Volume is important and 20 or 30mpw isn't much. Though 30mpw + 250mpw on the bike is much better than 30 miles alone.

Hard to give specific advice without more info.

i signed up for a 100km ultra marathon in april. running is my worst discipline of all 3 in triathlon. it's a first for me, but i'm hoping this gets me motivated through the winter and carries over into next year. i've never ran farther than a marathon (full IM). i've done some long zone 2 runs for 3.5-4 hrs in training this year, but that was at my peak. i'm also very concerned with injuries. i've done well to avoid them, but ramping up the training and distance slowly and being consistent are key. a few months into training, running on tired legs is where i think the building will be as well as zone 2. i don't foresee myself doing anywhere near more than 30 mile runs, but running on tired legs will hopefully help me replicate what mile 40 or 50 will feel like.

Thanks for the good answers:

As a bit more background. I'm 45-50, only been doing any sort of Tri sports for about 4 years. I've done IM Wales and a handful of 70.3's with a fastest of 5:10 for a 10th AG. I can run an open HM in < 1:30. or at least i could 2 years ago before an extended bout of plantar fasciitis buggered up my running. My previous high was only ever about 25mpw. Since then i've gone down to virtually nothing and now i'm back to consistently 15-25 miles (about half of it on trails).

So, i can do the long races, both physically and mentally... But i appreciate that in this game there is no limit to the definitions of "long" or "fast" and that the race can kick you in the arse to knock the flippancy out of you :)

Since i signed up for a trail marathon in March, a couple of weeks ago, and set myself the goal of doing an Ultra it's been a great motivator for the winter. I'm just about to put the headtorch on now and go for a quick run in the rain before work.

I'm not sure that I have much of a running base. But, I have always been a solid runner. I did HS XC for a couple years and College XC for a semester, but was never more than a 17:30-18:00min 5K runner if that. I skipped base training in college and just seem to wing it for most races. This was 2000-2001.

I didn't have any trail running experience and learned a good deal of navigation on the course, which wasn't difficult in the day, but proved difficult at night. I didn't think I was going to finish in the night and didn't prepare appropriately. I started to get the shivers at the end (the uncontrollable ones) and it was slightly scary. For the most part, the nutrition in the day was fine and it wasn't too hot so there was not a lot of loss of electrolytes. It was a pack your own aid station and I packed filled camelbaks on the aid stations with reeses and pretzels and pringles and stuff as well as gels. I didn't eat that much during this time - much less than expected.

I had quite a race experience and would highly recommend the 50mi distance. 100mi seems a bit much to me. I don't know if I'll ever attempt a 50mi again though - it was quite an effort.

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https://www.strava.com/...tes/zachary_mckinney

Increase your run mileage slowly. You should be running a minimum of 35-45 miles per week and you need to 3-4 months of this base. You don't need to go much higher, but consistency is the key. Your biggest help will be working on strengthening hips and glutes (do the Jane Fonda Hip Complex exercises), roll out and take care of any niggles before they become full blown issues. Once you have a base, I would have one day a week when you run a double. 10 miles in the am (6am) and 8 miles in the pm (6pm). You can certainly finish a 50 miler based on your fitness levels, but it will likely suck badly. Good luck!

Haha, this made me laugh.. In some crazy masochistic way this attracts me..... So i've gone and signed up for Race To The Stones (100km) in July 2020. I'm going to become a convert to S&C and do this properly!