For developing into a long distance cyclist, what is important : Cadence or Speed?
I'd say Cadence, always. Important too, since it ensures your knees don't get shot. You need to practise holding >90 rpm for extended periods. The one example where Speed matters, would obviously be in Track events, where you anyways cannot control cadence.
Cadence is a matter of technique - you train it for all types of riding. There is nothing specific about long distance cycling and cadence.
Speed - what about it? How will increasing your maximum speed from XX to YY help you cover long distance better?
In any case, don't over-think it: just ride. Build up your weekly mileage and do at least one long ride a week (30-50% of your weekly mileage), also gradually increasing the distance of this long ride as well.
a little too late to reply....but for long distance cycling managing your heart rate is more important 60% of mhr