Here's a bit of an example that should work...
If your variable for speed was speedx# and it accelerates from say, 0 to 2.0, then do something like this:
if speedx#>0.0 and speedx#<0.5 then Set Object Speed objnum, speednum (percentage)
if speedx#>=0.5 and speedx#<1.0 then Set Object Speed objnum, speednum(raise the percentage a bit)
and on and on
If you match it up just right with your actual code you can make it appear that with each footstop that falls his speed increases.
Like I said that should work, I have code sorta like that in my project. Set Object Speed can tend to be a little iffy at times though... best of luck!
Neeeeeeeewom!