Backpedaling is a valid tactic that comes with its own set of risks (more likely to get attacked from behind, sacrificing your speed bonus, might walk off a cliff or into a corner, etc).
However, persistent S-key spammers are annoying as hell, as someone that uses a short weapon may have to block 5-6 times just to get in range of a backpedaling glaiver. The best solution is a stamina bar (running backwards while holding a long weapon is way more draining than running forwards), but we know that won't happen, so a more likely solution is the following:
Increase movement speed penalty from weapon length while backpedaling. Weapon length is already used to impose a run speed penalty, so that code can just be expanded to include an additional factor that applies to backpedaling.