All distance are one way.

I think that 10 miles +/-3 miles is perfect. It gets a satisfactory workout in and is not too long. But that depends on the terrain. For years I did a 14-15 mile commute with some decent hills. That was exhausting.

Now my longest route is about 10 miles. I do not deviate my route much since I value my family time.

This morning was 7 miles, in the first 1/2 mile up hill it was too long. The last two miles on the flats with a tail wind it was not long enough.