Get yourself 4 3/4" elbows for 1" pvc pipe and buy four 10' lengths of pvc pipe and your local home improvement store. You've now spend about $12. Cut the pipe to about 7' lengths and build a square frame. Tie or tape a green or blue or red fabric panel to the frame. Come to think of it, go to a fabric store first and find an appropriate color bolt of fabric that's about 50 or 60 inches wide and buy two and a half yards of it. Now make your frame fit the fabric.
Get two of your friends to hold the screen about far enough behind your actor so he does not cast a shadow. Have your actor and your friends walk toward the camera while they maintain the same distance behind the actor. Don't worry about the feet, just make sure that his clothing and his shoes are significantly different in color than what he is walking on.
Bring the shot into AE. At the start of the shot draw a rectangular mask around the actor and the green screen including the actors feet. Set a keyframe for the mask. Move to the point in the shot where the actor's feet just start moving out of frame and adjust the mask to fit. Move to the end of the shot and make the last adjustment in the mask size.
Now you have an actor on a green screen with a garbage matte that hides all of the stuff that is hard to key. The next step is to pre-compose this shot then duplicate the pre-comp and use rotobrush to mask out the feet. Don't worry about anything but the feet. That's all you want to have in the frame and you only have to roto until the feet go out of frame.
Now apply your favorite keyer to the bottom copy of the footage. As soon as the feet leave the frame you can trim the top copy.
You now have a two layer sandwich that has only your actor walking toward the screen. Pre-compose both of the shots and use the new pre-comp as a track matte for the original footage. Do whatever color correction is necessary to clean up the shot and you're ready to render you DI (digital Intermediate) to a production codec that supports an alpha channel or your ready to add your white background.
That's the way they do it in the movies with it's not practical to have a big stage, even when they have big budgets.
If you want to get really fancy buy a 60" wide roll of background paper from a production supply house, roll a sufficient length of it out on the ground for your actor to walk on until his feet leave the frame, and have him walk on that. Then you only may not have to roto any of the feet. The whole point is that you can mask. you don't have to build a whole set.