45s seems too fast. Assuming you kept roughly the same pace, you’d come through around 61-61s. What 400m time do you expect to hit. At a target race time of 50s, a pace of 61 is 82% effort. No wonder they felt pretty hard. That’s definately not slow tempo in my book. I would think you would want to target times btw 54s and 57 to stay btw 65-70%.