John Frey once held the national record at around 52:00 early in his career with RGRT. I remember when it was still above the 50:00 mark around 1987 or so, then came the "fast years" from 1988 to recent. I think Kent was the first one to go sub-50 with his Ten-Speed Drive Guercotti, and within a year he got that down to 48:44 or so, when Frey did his thing in 1990 (47:35?). On that same day, Kent also went 47:40-something. I only recall one other person going sub-48 on the Moriarty course, and that was Colby Pearce with his high-dollar Lotus and Superman position. In recent years, it seems that the 50:00 minute barrier has become harder to break.
All that being said, I think the Brits were the first ones to make the 25-mile time trial a famous distance that was focused on for speed records. Our times compare closely with theirs, but it wasn't until Boardman came along that the 47-minute barrier was broken. Then there was a string of them as well.
Anyway, back to your original question, Mari Holden's time is pretty killer. Many top-level (Pro/Cat-1) men can't time trial that fast on an equitable course with the same conditions. That time will stand for a while.