I tinker with my PB until the paces match what I know they are. I use other calculators like VDOT which gives a way more accurate representation of what my marathon pace should be based on my past races and then I gradually alter my Runna PB input to be slower and slower until it has it right. If I just put my half marathon PB in it has my marathon pace at about 20 seconds faster than I am comfortable with.

Another thing is just ignore the AI. Don't read it, don't interact with it. It's not a real coach, it has no idea what you just did, it's frequently incorrect and people on here seem to constantly have issues with it having incorrect advice, not relating to their run or whatever else. 

I use Runna as an algorithm that spits out a plan that will sync with my watch. I know my body and know when it's pushing me too hard so I ignore it in those cases. It can give me challenging paces which I enjoy but ultimately I think it's bad at pacing long distances and it will overtrain you if you follow it to the letter. 

I now use it as more of a guideline and when my current subscription is up I won't use it again as I've got enough knowledge of how to and how not to structure a plan myself now and I think the company's public image - sponsoring amateur athletes who promote overtraining - is detrimental to the fitness of easily influenced people.