In this study they assigned people randomly to get either an exercise program or health education. So it's not a self-selected group of exercise fanatics vs couch potatoes, and so long as they track people assigned to the exercise group who drop out properly it should be valid.

I think the biggest benefit I get from exercise is psychological. If I am having a busy day and I skip exercise to get more work time it is almost always a mistake because I wind up spinning my wheels.