Dim start%, now%

...

If Val(Speed2$ < 3) Then
 now% =  Val(Mid$(Time$, 4, 4))
 If start% = 0 Then
   start% = now%
 ElseIf now% >= start% + 1 Then
   Print "Time is up"
 EndIf
Else
 start% = 0
EndIf

'Const DURATION% = 5 * 60 * 1000 ' 5 minutes - for reality
Const DURATION% = 10 * 1000 ' 10 seconds - for testing
Dim speed% = 5

Print "Speed" speed%

' Sit in a loop, "A" accelerates, "Z" decelerates.
Do
 Select Case UCase$(Inkey$)
   Case "A" : speed% = Min(10, speed% + 1) : Print "Speed" speed%
   Case "Z" : speed% = Max(0, speed% - 1) : Print "Speed" speed%
 End Select

 ' If speed has been <= 2 mph for more than DURATION% milliseconds then exit loop.
 If is_stopped%(speed%) Then Exit Do
Loop

Print "I have stopped"
End

Function is_stopped%(speed%)
 Static start% = 0
 If speed% >= 3 Then
   start% = 0
   Exit Function
 EndIf

 Local now% = Timer ' You don't use VAL() as TIMER isn't a string.
 If start% = 0 Then
   start% = now%
 ElseIf now% >= start% + DURATION% Then
   is_stopped% = 1
 EndIf
End Function

do
 if GP0 = 0 then getbutton
 'mode will alternate between 0 and 1 on each time GP0 goes from 1 to 0
loop

SUB getbutton
 mode = NOT mode
 do
 loop until GP0 = 1
END SUB