I worked with NPS, running a state historic preservation office (1983-2012). These "SHPOs" are funded with grants administered by NPS. In 2004, I was appointed to the National Historic Landmarks Committee, which reviews landmark nominations for NPS to consider and forward on to the Sec of the Interior. In 2009, I was appointed to the Advisory Board for the National Park System and to serve as the chair of the NHL committee. I stepped down from that position in 2013.

All of this gives me a good perspective on the historians employed by NPS. There are people who function as historians in the field: some have degrees and some are inspired rangers who are self-taught. Roughly half of the nation's parks are devoted to history, so there many opportunities across the nation. In addition, the National Register Program in DC administers the Landmarks and the National Register programs, including administering the grants to the SHPOs. That office employs many historians.

There is always the issue of geography. If you live near a park, I recommend you do whatever you can - volunteer or something - at the park to get practical experience. If there isn't a national park, seek out a state or local park that deals with history and learn the trade to gain practical experience while you earn your degree.

Best of luck!