Once the athlete reaches a speed plateau and seems to have difficulty moving to a new higher level. Then something different needs to be done. Flexibility improvement - Especially in areas that may decrease stride length. If the athlete has excess weight losing it will speed them up. Lean body weight should be at least maintained.
Excessive bulk is bad, as is loss of flexibility.
A program needs to be developed to suit the individual. The right program is needed for running the athlete has only so much time in the week and they need to recover for their high quality running sessions. The athlete could sprint up short hills, tow sleds, tow other athletes or run with a weighted belt. The most recommended way to do this is with a weighted belt. They are comfortable and can even be worn all day.