Houses, Stairs, Walls, Floors, Outside, Inside, if you're fortunate enough to possess a pair of legs, you have the ability to walk, or run! Of course, pick good times to do these things, don't run in a crowded space where you're liable to run into things or people and piss them off, but do it on empty or near empty streets, fields, etc. Running doesn't require much effort really, and is a great way to build your leg muscles!
I find it helps to run or even walk fast (eh, whatever gets you sweating is good :) ) if you have music with a beat. I recently bought the "Ministry of Sound: One" album which has 3 CD's and is great for this type of thing, if you have an iPod, MP3, MP4 player, etc, whack this album on it, start from #1 and work through it, it's great! If you're not into this type of music (HOW COULD YOU?!?!) then find something with your equivalent and post it as a comment! :)
My Personal favourites to run and walk to are:
Chemical Brothers Hey Boy Hey Girl Underworld Born Slippy Dizzee Rascal Bonkers Fedde Le grande put your hands up Benny Benassi Satisfaction The Shapeshifters Lolas Theme Alice Deejay better off alone
Etc, these with regular beats are good, as you an put your feet down on the beats, so walk to the beat, run to the beat etc