On Sunday, I rode 7 miles, but kept a cadence somewhere around 100 rpms and felt very very tired afterward. Advice?
Take it easy.... The great thing about cycling, is that you could do it all day, if you just "take it easy".
Use your gears in a way that keeps your knees from hurtng. Usually it's better to ride longer and slower for general fitness. You can pedal easy gears, without spinning "high rpms".
Take a while to get used to riding, then try for harder work......