Wait...your toenails can fall off if you run regularly?

Yep, I lose them while on long downhill hikes. Get shoes with a larger toe box and keep toenails short, that helps.

I am about 50/50 on whether or not a given marathon or Ironman will claim one of my toenails.

I reckon having the right shoes makes the difference.
On my second half and they're all in good nick still. At a much higher training load it may be different.

My toenails never fell off from running.

I never lost any doing 30 minutes of running regularly. But I was fitted with a men's running shoe instead of a women's, even though I am a woman. Apparently my size (UK 7) is only available in men's in some trainers. But it was good because the toes were a bit wider and a lot more comfortable that the women's.

Agree with those who mentioned shoes.
If shoes are too small, the toe nails will suffer (as well as the rest of your foot). Have shoes properly fitted. The toe box has to be roomy; toes away from the sides of the shoes. Running shoes are at least a 1/2 size larger than your regular shoes to keep the feet comfortable as they expand. This protects the nails and keeps them from turning black & falling off.

I lost some toenails when doing ballet, and one to trauma from two of my kids stomping it accidentally within the span of a week, but none from running, so far. My feet are narrow by today's standards, though they measure a true B, so I have to buy shoes that are not too long (if they are long enough for the 'one thumb of space at the end' then my feet will just move around too much in the shoes, I can't secure them). But narrow feet means toe boxes are usually too deep for my feet even then, which probably saves the toenails.

I do, however, take an 11 in running shoes usually... I think if trying to get into a shoe longer than my foot, I'd run out of women's sizes altogether, and men's shoes would definitely be too wide and deep.

I've run for 40 years including a couple marathons, never came close to losing a toenail. Never even had one discolored.

Watch the fit of the shoes.

I've never lost any toenails. I've done 4 half marathons and one full. I haven't ever had a blister either! Good shoes, fitted to your feet, are essential. After my most recent half, one of my toes was sore but I had switched shoe brands. I have since gone back to my original brand!

I've been running for something like 10 years, and I've only ever lost one toenail. This was because I was wearing shoes that were probably half to a full size too small, so my toes just got crunched in the toebox. Ever since I figured out my true running shoe size, I haven't had any toenail issues.