Here is another alternative: Vary the period you increment your speed.
This examply uses PAUSE.
Code:
Incr_Time_ms = (1000 * Duration) / (End_Val - Start_Val)
Current_Val = Start_Val
Do Until Current_Val = End_Val
   PAUSE Incr_Time_ms 
   Current_Val = Current_Val + 1
Loop
What would be ideal is set up a timer and add a value so that it overflows after "Incr_Time_ms". You can then use interrupts, or just poll the flag bit. increment your "Current_Val", clear the flag bit, and then add the value to set up the overflow after "Incr_Time_ms". Wash-Rinse-Repeat until "Current_Val = End_Val"