Without looking at the guide you were reading, I don't know for certain what it says, so I will answer the question of "steady dating" independently of assessing what that author was trying to say.
Steadily dating someone for a long period of time when one has no intention of marriage in the near future is not a sin but it is imprudent. The purpose of dating is to discern marriage. If your individual circumstances mean that you are not a position to be considering the vocation of marriage in the near future -- and twelve to eighteen months is a reasonable timetable -- then you should be wary of entering a committed relationship with one person.
That's not to say that you can't casually date a wide-range of people, as do older teenagers and young adults, with the intent of socializing and learning how to interact positively and constructively with the opposite sex. It is only to say that it would be imprudent for someone with no current interest in marriage to become involved in the type of relationship that is expected to either progress to marriage or end. One reason it is imprudent is because "steady dating" encourages the kind of physical intimacy that is appropriate only to those who are engaged to marry. Regularly engaging in that kind of physical intimacy with no expectation of marriage may lower one's inhibitions about chastity outside of marriage and place one in the near occasion of sexual sin.
If You Really Loved Me
by Jason Evert
Christian Courtship in an Oversexed World
by T. G. Morrow