Higher cadence or lower cadence for long distance cycling I am going to ride long distance like 200 km on my hybrid. I am not sure if I should pedal in a higher cadence (a lower gear, pedaling more frequently) or a lower cadence (a higher gear, pedaling less frequently) for the same distance.  Or should I mix between both?

Can anyone please suggest?
100 RPM minus your age.  (Only half kidding.)

80-90 RPM is a good target for younger, fairly serious bikers.  When I was in my 20s-30s I could do that for several hours.  As I get older (I'm 63) I find it harder -- 70 RPM is probably closer to my "optimal" speed now, and I drift down toward 60 if I don't keep at it.

One rule I tell folks that I think is good for almost all environments, whether flat-out or casual, uphill or down, is to never pedal slower than you're breathing.  Keep your cadence at 1-2x your respiration rate.

If you're riding along lazily with a resp rate of 30 you can get away with a cadence of 45, but if you're resp rate is 60 your cadence should be at least 60, maybe 80-90.

Similarly, if your cadence is 90 and your respiration rate only 40 then you should be using a more difficult gear.