The treadmill can be your friend when the weather and/or the terrain in the great outdoors is not conducive to running. Before heading to the treadmill consider a few points.
A) Not all treadmills are the same. In fact, without proper calibration, chances are very good you will not be able to exactly match the pace you and distance you normally can accomplish when running outside.
B) The indoors can be warmer then outside. Be sure to adjust for the temperature changes.
C) Do pre-workout prep work before getting started. Have a plan. Make sure you have what you need. For example, hydration, devices, headphones, etc.
D) Have a workout ready for you . Typically, being mentally prepared for a treadmill run will make the experience. That goes for the shorter runs to the longer runs.
E) Lag time... Very important to recognize the lag time it takes to speed up and slow down. This especially applies to speed workouts.
F) Wind resistance: Since you are running inside, there will be no wind resistance. To adjust for this, you can play with the incline based on pace of your run. The general rule is Zero percent incline for paces over 8:30 pace. For paces 7:00 to 8:30 pace, a Half percent would would be good. If you are faster then 7:00 pace, a One percent incline works.
G) Your foot plants are going to be the same every time on a treadmill versus the different terrain and slopes you would face on an outside run. For some this is not a big deal. For others who deal with the occasional nagging injury and such, this can be an issue. With that, just be aware of this and know you can play with the incline a little more. You should also adjust the speeds of the workout to overcome this as well.
These are just a few pointers. I could get really granular here however lets stick to the basics.
Just remember, the treadmill is a great alternative to running outside. Many athletes dread this piece of equipment. And while it can be boring and perhaps laborious, having the ability to get a workout in when the outside conditions would otherwise prevent that from happening, it an awesome thing to have.
So let's embrace our "friend" versus the alternative.