This is a great question! I inherited a family pan that's been in my mom's family for almost a century and it was pretty rough on the inside. On a side note, it's interesting to see the pitch and char that's accumulated on the outside from being used atop a wood burning stove for many of those years. Here's what I did: Heat up with a couple of tablespoons vegetable oil in it. Spread the oil evenly and wait for it to smoke. At this point, you should notice that some of the sticky areas are sizzling. Take a thin metal spatula and scrape that crud off and toss it. Keep this up until the inside of the pan is of a uniform sheen and smoothness. With the pan still on heat, wipe it dry and add some more oil and spread evenly. Once the fresh oil starts to smoke, turn off the heat and allow pan to cool completely. If you season correctly, the oils actually form polymers on the surface of the iron that are fairly resilient. Despite the myths, you can use soap and a light scrub to get stubborn food out of your pan. There is a gread article on the Serious Eats website by a chef named Kenji Lopez about cast iron care. I suggest reading that.
Use it in good health, friend. Cast iron is fun and delicious! Now, off to pan sear some meat.