the runtime program is actually a full version of Access, just disabled internally so that it only runs as a runtime
I am not sure if MS provide a runtime download only
otherwise, the installation packager includes all the necessarily files (support files) to install the runtime - its not just files, there will be lots of registry settings, folders created etc - I doubt it would be easy (maybe not possible) to install the run timer manually
It's not that big - the packager should prepare a self installing run time system, all on a single CD
Note also that as the runtime is really just the full version of Access, if your users subsequently buy Access, they will then get full access to whatever your database includes - so although in the runtime they will not be able to see/edit any code, if you include code unprotected, they would be able to see it with the full version.