Hold B to Walk

If you want to save a few CPU cycles, you can actually modify it a little bit further:

Code:

MODIFIED
$C0/4E21 AD 19 42    LDA $4219   ;Load 2nd byte for controller 1
$C0/4E24 29 80       AND #$80    ;AND for B button only
$C0/4E26 D0 0B       BNE $4E33   ;Branch if button pressed/held
$C0/4E28 A9 03       LDA #$03    ;Load sprinting walking speed
$C0/4E2A 80 08       BRA $4E35   ;and branch
$C0/4E2C EA          NOP            ; Not technically necessary, since
$C0/4E2D EA EA       NOP : NOP      ; this code would never be reached, but
$C0/4E2F EA EA       NOP : NOP      ; visually cleaner than leaving it in.
$C0/4E31 EA EA       NOP : NOP
$C0/4E33 A9 03       LDA #$02    ;Load normal walking speed
$C0/4E35 99 75 08    STA $0875,y ;Store walking speed

This removes the check for the Sprint Shoes bit, since you're not using them anyway; hilariously, if you WERE using the Sprint Shoes, the change you made would cause them to have the same effect as pressing B -- your character would walk slowly.