I ride 10,000 miles a year, my average daily effort is around 35 miles, and I've never found cadence to matter one single bit. Unless you're turning like 40-50rpm, self-selected cadence is usually what works best. As cadence gets higher and higher, it becomes increasingly inefficient.
What will leave you tired after rides is riding at a level of intensity that is unsustainable at your level of fitness. Some days simply need to be of lesser intensity, and most days will be of 60% effort or less.
The bottom line is, if you want to ride more miles, you need to ride more miles. Longer rides, more often. I can't even get my legs warm in 10-15 miles.
__________________