You will only damage the skin surface. Dont waste your time striking with much other than punching. Those calluses mean that you are hitting it wrong. Do not try to induce this. Some people's skin won't do it.
Some people only get it on one and.
The makiwara is the core of true karate training. It is the most important element. Skip anything else when time is limited and hit the makiwara. Water seal the base of the makiwara before you drop it in the ground. Don't ever use pine to make a makiwara.