In this case scenario, I would recommend setting your athlete type to something a little further up the PD curve (maybe Climber or GC Specialist). When athletes are preparing for those long distance events, I would argue that the focus duration becomes less meaningful than the athlete’s training load, which is built up through Improvement Rate.
Moving your athlete type further up the PD curve will help you perform better in those Zwift races. In the meantime, you’ll still want to continue building that training load as high as you can prior to that 300k sportive later this year. A general rule of thumb is to aim for a Training Load of 1/4 to 1/3 of the expected XSS of your sportive.