Updated: Aug 14, 2019
First, running on the treadmill is often easier than running outside for numerous reasons:
Running on the treadmill assists with leg turnover, making it easier to run at a faster pace. With that said, your pace on the treadmill probably won't match up to your road pace.
The treadmill provides "give" on the base which means some of the soft tissue conditioning or “hardening” that occurs on the road is not seen on treadmill running.
Weather is not an issue on the treadmill.
Running or walking with the treadmill at a slight incline reduces impact in the joints and vertebrae.
The treadmill can be a great training tool if used properly. Use the treadmill to help you run faster by doing intervals, speeding up the pace for short intervals and then slowing it down for recovery intervals. This is a very convenient way to get in some speed work or tempo runs in a controlled setting. Intervals on the treadmill can be a great way to burn fat.
If you enjoy running, a program to follow is to run outdoors two or three days a week, with one day designated as your long run day and one or two other runs of shorter distance. This will provide the conditioning and the weather tolerance for athletes that require outdoor running. One or two days a week include the treadmill for speed work or tempo runs. This gives you three to five days of running with multiple disciplines. Add some stretching exercises to promote muscular flexibility and you have a very well-rounded training plan.