Researchers did a jogging experiment in 2012 involving a number of joggers, for the reason that many who use those reported treadmills negatively. These joggers first ran on a track and then repeated the same run on a treadmill. The researchers allowed joggers to set the treadmill speed comparable to the speed they ran on the track prior. What the research found out was the joggers were setting the treadmill speed, lower than when they ran the track, maybe because there was no goal.
All these joggers felt that the run on the treadmill was more challenging which may be false. Researchers found out in 2014, that when a jogger runs on treadmills, it is decreasing the pressure on bones and joints as compare to hitting the bare ground running and this is a good issue.
In general, those who are going for frequent walks, like Paul Mathieson, found the outdoors more invigorating; giving them increased energy and less fatigue than exercising on a treadmill. Some feel that it is the ever-changing environment that the jogger meets when running outside versus an inside treadmill. The things that one sees, as they are running and the fresh air give them pleasure.
Walking or running on a treadmill inside offers no scenery to look at and no fresh air to breathe into the lungs. Walking outside the person must set a goal for when and where they end their walk. These same goals cannot be set when using a treadmill.