Heart disease is largely genetic and so is blood pressure. There is nothing that is "enough" if you are predisposed to it.
Cardio workout is helpful for heart health but that is not the same as a cure for heart disease which has many factors.
Cycling can be useful for cardio workout but the way most people do it it is completely worthless for that purpose. Cardio workout needs to have very intense heart rate elevation for moderate periods to have a strong effect.
So that is why my other idea for a thread was kind of dumb. You can't really talk about cycling for blood pressure or heart disease, especially not in terms of distance. You can talk about it for cardio workout but to really get a good idea of how you are doing then you most likely have to get a heart rate monitor and time out your rides. And even then when you are sure you are getting optimal cardio workouts, you may get very little BP change, or very great BP change all depending on what is going on in your body. And BP itself does not guarantee a healthy heart either, though it will for people who don't have some underlying heart issue.