Always remember: Running is a Skill and the longer the race, the more skill, strength, endurance and conditioning you need.
With proper preparation, a marathon or triathlon can be one of the most gratifying experiences that you can ever have.
Your goals should include improving running economy and performance, maintaining muscle power (particularly as the athletes mature), as well as preventing injuries and long term postural deficiencies though overall strength and flexibility training.
Logging in too many miles plus a lack of overall strength and flexibility can create long term motor skill problems as the body continually adapts to find the path of least resistance and turns away from proper running mechanics
When preparing for for a marathon, triathlon or other distance race, the initial assessment will find out what your strengths are and in what areas the client needs to be stronger