As it turns out, the above was exactly right. I went for a ride today and tried to keep my cadence at 80rpm instead of 90 and went from being able to peddle maybe 50% of the time to being able to peddle close to 95% of the way round my 6.16 km ride today. This meant I worked up a mild sweat as opposed to getting home drenched and exhausted.

Frankly, I'm stunned at the difference dropping 10 rpm made. My legs were far less exhausted and burning and I was not exhausted when I finished. I can probably already add distance just with that one change.

Thanks for the advice everyone!