Hold modifier key to Walk/Run, rather than toggle

[bradleypariah]

Title pretty much explains it. I'm a n00b, but I'd be willing to attempt this on my own.

If someone else can make a mod that allows the user to hold a key to walk, then let go to run (or vice-versa), that'd be awesome.

If you can point me in the right direction to making such a mod myself, that would be appreciated just as much. I'd be happy to share the mod once it's made.

I'm a sound guy. I know Wwise and implementing audio through engines, so I understand video game development and tiny little bit of coding, but movement behavior modification is not something I have ever come close to attempting. It's safer to assume I know next to nothing.
I would love a link to a good tutorial.