Q&A: increasing mileage safely/black toes/running stability/water running

If we are trying to ramp up mileage extremely quickly for a race without getting injured, how can we successfully do so?

I’ve heard runners get black toes. What is it, how is it prevented and how to treat if you get one or more?

How to keep your runs focused when not working toward a specific event/race?

Stability work for runners, particularly trail running would be awesome- how important it is, and how can we put it into practice?

Is water running a good substitute for land running when you are injured? Does it have any strength benefits? Is it a worthwhile exercise?