It seems that my increase in running mileage has prompted my big toe to get all bruised, painful and slightly mushy. Apart from stopping running, is there anything I can do to prevent the nail from falling off or getting worse?
Luckily it’s winter and not open-toe shoe time of year!
Get better fitting shoes.
I’m not sure if it will stop the loss of the toenail that’s already started down the wrong path, but it should keep the rest of your nails from following suit.
If it’s got blood under it you can burn a hole in it to let it out and that might help. Other than that, I got nuthin’.
Yeah, even if your shoes feel fine you may find that a different brand eliminates the toenail problem. I had been faithful to one brand of shoe for many years; when I tried another I got the black toenail even though the new shoes were otherwise perfectly comfortable. Switched back to my old brand, no more black toenail.
ETA: Unfortunately I don’t know what different feature to look for in a different brand–larger toe box? Differently shaped toe box? A good running specialty store should be able to help you.
Once it’s detached I doubt that there is any recourse but to allow it to slough off. (Eww!)
A thought: Take care to keep the area clean. I’m guessing that there is a risk of acquiring a nail fungus if that is not what is already occuring for you.
Once it’s black I’ve always had it eventually fall off. It’s been almost a year since my one toenail turned black from running and it’s still not fully recovered (basically is now though). Prevention going forward, getting a shoe with a larger toe box, trimming your toenails (I cut mine basically 2/3 times a week now). For longer runs, slap some vasoline or body glide on top and around the toe nail.
First, losing a toenail isn’t that bad. It seems gross but having it come off gradually (as opposed to torn off when you stub your bare foot on a table leg or something) isn’t traumatic. If it’s really going black or getting loose or something, toenail loss is in the future so see a doctor and get it handled properly (they might trim it off right away and tell you to keep it clean/dry/use antifungals until it grows back, for example). At least the first time (and hopefully only time!) get some proper medical advice on how to care for that injury.
To prevent it in the future, definitely get proper-fitting shoes. I recommend going to an actual running store staffed by people who really run (skip the local big-box SportMall kinda place). You need enough room so that your toes aren’t smashing against the front (or top) of the shoe, for example. Running downhill moves your foot forward in the shoe so you want to be sure you can lace it properly to prevent your foot from sliding around and that there’s adequate room to handle this kind of thing (and the fact that your feet can swell up a bit if you run for a long time, thus changing the fit of your shoe temporarily).
Take care of your feet - keep your nails properly trimmed and smooth for starters. That helps to prevent the nail from snagging on socks or anything and getting yanked.
Wear proper socks (which means no cotton at all).
There’s a wonderful book called “Fixing Your Feet” by John Vonhof which I highly recommend. It’s written for long distance runners but the same rules apply whether you’re going 5 miles or 50 miles.
I stubbed my big toe in June of 2007 and it took about a week to fall off. It wasn’t till May of 2008 when it grew back in fully
As a runner, I have lost both big toenails several times. For me, once it turns purple, there is no hope—it stays purple for a few weeks, then the blister drains one day, leaving it a pallid yellow white color for a few weeks, until the day when it finally comes off (usually with my help).
Never is there pain in this process, except perhaps a for dull throbbing the day that it turned purple.
I have posted pictures of the peeling process in the past, but will exercise restraint and will spare you that unpleasantness 
Get shoes with a bigger toe-box and take it easy on the downhills
Wherever I go on vacation I spend most of my time walking or hiking. So every summer I wind up losing my second toe on each foot (they are the same length as the big toe). I wear excellent hiking shoes, but no matter what shoes I’m wearing the problem persists. But the nails always grow back. They used to take six months, but now that I’m diabetic they take nine.
Really? I thought cotton socks were good for your feet. (FYI INA runner.)
Great for a blister.
Question–just how much running does it take to do all this horrible damage? I wear cotton socks when I run and I’ve never gotten a blister or a weird toenail injury or anything. Do you have to be the kind of person who does like ten miles a day? Because I’m so not giving up my pretty feet for that.
It must be the toe box. I will check into new shoes.
FYI, I did get fitted at a pro store (Running Room) but did change brands this year (New Balance to Saucany). I bought them last summer and used them all fall, but I did recently start increasing my weekly mileage so that must be the factor. I suppose it’s time for new shoes anyhow - I was just hoping to make it to the spring.
Freudian Slit - I’m running about 20 to 25 miles a week right now and have increased to this distance recently (and will continue to increase). YMMV however. 
As the others have mentioned, it’s the shoes. Fit should be snug all around with clearance at the front(thumb width). You don’t want your foot moving in the shoe. Your feet will swell during a run so check fit after a run.
You won’t save the nail but be proud, you are undergoing a rite of initiation. 
runner pat, who has never had a blister or black toenail and hence is not a real runner.
25-30/wk (though work is getting in the way of this recently)
My main problem is that I wear size 14, so good fitting shoes are somewhat hard to find. In the middle sizes, there are lots of variations and such to choose from. Usually in a 14 you get to pick from two pairs of shoes at best. Even if they have enough room, there is always some tight spot.
After a few of these incidents I went to a running store in Princeton (Princeton Running Company on Nassau) where they watched me run, inspected the wear patterns on my old shoes, and selected a particular shoe. Ever since, I buy my shoes there instead of online since they have the selection and the expertise to give me the optimal shoe.
Even with the best shoe, black toenails still happen occasionally—right now I am missing the right big toenail.
Well, it was the toe box. I wore my shoes from last year, which are New Balance, for my run this morning and no pain at all. However, I now have a nice blister on the bottom of my big toe on the other foot. I can’t win! Haha!