Sprinting at the end of a road race is different to track sprinting and therefore requires different training. Try intervals of varying lengths from 30 seconds to 2 mins and go as hard as you can, accelerating every time the speed drops. Alternate sessions between long (5 mins plus) and short (1 min) recoveries between intervals. Long recoveries will mean that you develop sprint ability. Short recoveries means that you will develop fatigue resistance. Make the sprints as realistic as posiable (eg start at a high speed by rolling down a hill or coming off a wheel and aim/surge for a finish line). Remember to use a realistic gear and as you start to slow, work harder! Practice on flat, slight hills and steep hills. You could use a pyramid or fartlek session to encorporate all of the durations that I have suggested.