So was it the loss of salt or the hills that caused her calves to give way?

 

Probably neither.

 

Typical probable cause for nOOb marathoner:

Too few overall training miles

Over aggressive goal

Over aggressive pace

Trying to bank time

Battling injuries and not adjusting goal

Starting too fast

Not training in a way that adequately mimics course conditions (weather, terrain, hills, etc.)

Agreed.  The only thing I would add is if the weather was significantly warmer on race day.  At my last marathon, it was 61 degrees at the gun which isn't too bad but the humidity was literally 100%.  A lot of people tried to run their goal pace on that day and I can guarantee that none succeeded.  Looking at the splits, there were a lot of people crashing and burning.

Here's a nice article from Matt Fitzgerald.

How Much Salt Do You Need?