I have my first DOAN Magnesium Firestarter and it looks pretty gnarly after about 21 or 22 years. The insert has never fell out and it has started a lot more fires than the gnarled magnesium block would suggest because I stopped using the magnesium for quite some time.
I had to learn through trial and error a long time ago. Hahaha!
In my opinion and experience, you should have a supply of tightly packed, obviously dry, cottonballs on you at all times. If you have a knife sheath with a pocket, more in there in some type of smallish ZIP-LOK baggy... The actual block of magnesium, for the most part, is a nice grip to use the ferro rod on.
Some time ago, people were reporting in here, or elsewhere, I cannot remember, that the Coughlan (however it is spelled...) brand mag block and ferro rod was inferior with the ferro rod coming loose, etc. I don't know if that has been corrected or not. The DOAN...I don't know if they are still manufactured.
If you have cottonballs that are dry, or if they have been treated with Vaseline/PJ, if you have all of your fire planned out and have excellent tinder, kindling and small-medium-large fuel ready and it's constructed properly - one of these ferro rods could last you a lifetime because you're talking about a two strike fire except under the worst possible conditions. Two strikes and you have fire.
Use the magnesium when you have to but learn to start the fire with cottonballs, dryer lint, 0000 Steel Wool and possibly charred cloth. Then move on to the excellent ideas of Vaseline/PJ, paraffin and the little fire straws.
You learn to start the fire with the focus on fire construction and the bare essentials you can carry and then when you add PJ and other things, perhaps when you are injured or shaking from cold, etc., it will be easier for you - having done this so many times...