Don is right. Through the 1960's, the fullback got more carries than the halfback on most teams in what was called the "pro set." The Raiders featured the fullback as the main runner until they drafted Marcus Allen in 1982.
This changed when the "I Formation" became the favorite formation in the 1970's. I believe Riggins ran out of the two back set with the Jets from 1971-75. In 1976, George Allen gave him a mammoth contract with Washington, and proceded to use him pretty much like fullbacks are used now.
Jack Pardee moved Riggins back from the line in 1978, and he went over 1000 in 1978-79. In 1982, Joe Gibbs placed Riggins at tailback in the "I," and he was MVP of Super Bowl XVII. John Riggins never really had an ideal situation until he was 33 years old.