Odds of Hanging On
This is a career that you're expected to hold for a long time once you finally have it. Uncle Sam employs mine inspectors across the country and, while mining may not hold all the unemployment answers it once did, there are still plenty of mines in operation around the country.
Hanging on shouldn't be an issue. But if you want to excel (and also graduate from GS-7 pay), then you have to continue learning and demonstrating knowledge in all aspects of mine safety.