I disagree with the statement in the original post that if you could hold 300W for 45 min you could ride away from a Cat 4/5 road race field. Maybe you could if you were light and the road was going uphill. Or you could on flat ground if you first put out 700-1000 watts for 10 seconds to get a gap, held 450-550 watts for a minute to grow it a bit, then settled into 300W. This is quite different from doing a stead 300W for 45 minutes. And even so, most 4/5 fields around here will work together and drafting is significant enough that they would pull someone back doing 300W.
I had a friend who was a pro triathlete (sub 9 hour Kona-type guy) who started bike racing. I want to say his FTP was 350w, and he could motor all day long at that. Of course, he could not cross 900w to save his life - he just had no top end. Anyway, he rode away from every race field and solo'd for the win in crits and RRs until he got up to a 2.