In addition, based on the let's play video, it seems like it stacks with multiple units. I know that other spells can stack twice in some cases, so I am not sure from your screenshot if thats the case.
So if you have 3 heroes with tireless march cast once on each, you end up with 5 movement points. This is worth experimenting with in any case.
This is true, thought the devs where aware since its been up before 
(Or I am imagining things, wouldn't be the first time 
)
Pain in the ass to see computer players poke armies with 5 move into my face 
Sincerely
~ Kongdej
