Complex sentence: Walton's first novel was published in 2000, but even before that she was active in online fandom, especially in the Usenet groups "rec.arts.sf.written" and "rec.arts.sf.fandom".
Split into simpler sentences: Walton's first novel was published in 2000. Walton was also active in online fandom, especially in the Usenet groups "rec.arts.sf.written" and "rec.arts.sf.fandom".