There really is a basis for the old saying, "Too many irons in the fire". As a blacksmith I've experienced the results of not listening to this warning, "up close & personal".
When I help out at the local History Center during "Farm Days" , they bring school buses full of eager faces to our forge. These days most kids don't have any idea what a blacksmith does, so I usually explain the concept like this. "When it's hot, you can do everything with iron that you can do with clay, ... except touch it."
In the "olde days", before blacksmiths had thermometers, they found that the color of the iron changes with the temperature. It starts out with "black" heat (about 800 degrees F, and plenty hot enough to burn yourself on), and progresses through the color spectrum of blue-purple-red-orange-yellow & tops out at "white" welding heat (+2000 degrees F, just before it burns up). At the right temperature, metal becomes very pliable (about 1,300 degrees F, orange-yellow).
When you're working on metal (iron/steel) in a forge, basically, you put the cold metal rods into the forge fire, and wait for them to heat up to a pliable temperature. Then, ..."strike while the iron is hot." (Another tried-and-true blacksmithing proverb.)
If you try to work the metal at a lower temperature, you end up having to pound twice as hard, to get half as much work done. On the other hand, if you let the metal get too hot, ... it will simply melt and/or disintegrate.
The phrase above, refers to the problem of putting too many rods into a fire, so that you can't keep track of what stage of heating each piece is at. You can only work so many pieces, before you start burning some up and wasting your resources. There is always an optimum amount of work that you can handle at any given time, and still accomplish your goal.
This adage is aptly evident these days, referring to the overloading of our schedules with way too much stuff to be able to finish in the first place. We start out our days with "aspirations of grandeur", sure we can accomplish super human tasks, like that Japanese guy on "Heroes" who can bend time. If we just keep moving, ... we'll get it all done! (The accident division of the insurance industry, could probably give you some good statistics about this approach.)
I have a friend who is constantly on the move, ... physically and mentally. After spending the day with her (and based on the exhaustion I experienced trying to keep up), I was surprised at the end of the day how much of her original "to do" list ... wasn't done. She had however, managed to break 2 glasses, forget about the laundry in the washer, and add 5 or 6 other things to her ever growing list.
She was full of energy and good intentions in the morning, but somewhere in there she lost track of her "line of sight". The detours and delays, created frustration to her already overloaded system. (As well as the cut on her finger, from using a kitchen knife as a screwdriver.) After watching her struggle, I was reminded of this ancient forging phrase.
In blacksmithing terms, it's the equivalent of starting out in the morning to make some horseshoes. You find you have to stop to make a punch, try to remember to order more coal, have several farmers drop by to pick up the tools you were fixing for them (with appropriate chitchat), ... and finishing your day with about half of your iron ruined (still having a shoeless equine).
If you overload your schedule, even if it's based on necessity, you will find that you don't really accomplish your purpose. You must base your day on workable goals, and build in some flexibility for when the world decides to interfere with your plans (because it always will). Remember, ... "Don't put too many irons in your fire!".