Darning is quite easy, and nylon actually helps the wool to wear away rather than strengthening it.
Proper woool ought to felt at the wear points and that makes it more resistant to abrasion.
It's actually a better repair to unpick the damaged heel and reknit the worn out piece than darn. There's a type of stitch that doubles the thickness at heels and at the ball of the foot if needed that actually encourages the wool to felt.
I can show in person but no idea of where to find on line instructions. I'll have a look though.
Hey, whole new carving ideas........we need darning mushrooms and eggs
atb,
M