Alt + arrows creates the necessity of holding the key and also alters all keys, so You will have to make all other bindings work regardless of whetver alt is held or not. I've been playing DOOM recently and found it quite nice to have shooting on Lshift and strafing with Z+X, and interacting on del (which, on my laptop's keyboard, it adjacent to the arrows). In this combination jump could be left on space, or maybe Lctrl could be better...

As for fast movement, why not make an "autorun" button? This should not replace the "hold-to-run/walk" button, but a toggle button could be quite nice. And if You mean fast movement to be used only in certain situations - well, if someone will want to speedrun, they can arrange their keys so that holding "run" will not hinder them, or put something heavy on the key - not having autorun will not be a problem. An in-game solution, like stamina, would be required.