the runner needs to be a bit bigger then the rest of the runners for the proper effect.
You need to isolate the runner from everything else, then scale the footage up.
To isolate the runner, you'd need to rotoscope to remove the background. If you have access to Rotobrush, introduced in AE 10, it may make life easier, depending on the nature of the footage.
In AE's online help, there's a page of rotoscoping resources. To find it, go to help and do a search using the word rotoscope.