I think I've mentioned before that sometimes I run in the house. It was pouring rain this morning and I wanted to run, so I did it inside. My house isn't very big, but I can do a circle through my kitchen, dining room, and living room. We built our home wheelchair accessible so there is plenty of space between the cabinets in the kitchen and around my little "track".
Running inside must be a lot like running on a treadmill. It is much easier than running outside. I used my little fitbit to track my distance, via my steps. I ran for 35 minutes and did three miles. I felt like I could have run for 10-15 more minutes. It is that much easier.
I may allow myself to do an inside run once a week. It is a good workout but not as hard as running outside.