How you feel about running is what matters

The biggest differences between treadmill and outdoor running aren’t in how you run, but rather in how you respond to the experience.

People probably run slightly differently on treadmills: when you look in detail at the biomechanics, you find subtle differences in things like knee angles and peak ground forces, but the overall pattern is that the two movements are close enough that you don’t have to worry about the differences, as long as the belt is properly calibrated and not too soft.