I've gotten into cycling (very) recently as a fun way to get some exercise, after not having done any consistent fitness routine in a number of years. Note that the below is affected by being a ~300 lb clyde, though weightloss isn't even in my list of goals except in as much as it would make riding the bike less work. Basically I'm saying I'm interested in improving my cadrio fitness and being able to ride longer distancing, rather than anything else.

In the two weeks since I've gotten a bike, I've built up to ~6 km daily but I would like some help with which goals to prioritize vs the others to build my endurance.

My first issue is that while I can do 6 km, I'm still far from the point of being able to maintain an average cadence in the ~90 rpm range. Right now its more like I'll pedal in that range for several seconds and then coast for a bit, and repeat.

My second issue is that no matter how low a gear I choose, spinning at 90 rpm for any length of time will ultimately raise my HR above able to have a conversation rates.

So, my question is this: I want to build up the distances I can ride, but as I understand it, I'd be better off building my ability to pedal constantly over the distance I am already doing before I worry about going longer distances. Also, from searching this forum for endurance questions I've learned that the consensus is that building endurance requires keeping yourself at a lower HR, but I cannot spin 90 rpm and keep that HR.

Does this mean I should prioritize pedaling constantly at a lower (say 60 rpm) cadence over the distance I'm already doing, or should I prioritize going longer distances and not worry about the pedal-pedal-coast issue and maintain 90 rpm when I am pedaling?

Basically I'm asking how to weigh the priorities of HR vs average cadence vs distance in order to build all three as efficiently as possible?