Demanding running workouts in very early pregnancy?

Hello, I got my positive pregnancy test last Sunday <3 And today I am officially late on my period - technically I'm at five weeks since I have long cycles but really it's more like four weeks? I'm in the middle of a fairly demanding half marathon plan. Today's work out is 6x1000's at 3-5K pace. I will definitely have an elevated heart and be a bit breathless! Considering I'm so early on and don't have much in the way of symptoms yet, do you think it's okay? I understand I will need to slow down as the pregnancy progresses but is there any reason to start holding back now?