Their first marathon should be exciting and it should be about not getting first place but just surviving it. They have to train really hard throughout the year and they would have to be really committed to the goal. They would ultimately start training after Christmas and build from there. They should build it up by 10% every week.
Your client would have done a few half marathons and some shorter runs to get to marathon.
Have them keep a diary and share their trainings and learnings with you. You can do it online or in a physical book.
Have them track their food- it doesn’t matter about the calories but its about the macronutrients and vitamins and other things that the body needs to recover. They need to understand their energy needs, ie for a female its different than a male as they would have menstruation to think about. If a female doesn’t consume enough she could lose her period.
Your client needs to understand the different supplements and how to use them for their runs. They need to understand how to take the energy gels and chews and they should practice during their long runs.
Your client needs to understand their hydration needs and practice it. Do convince them to have about 4-5L per day. It may take some getting used to but you’ll get there in the end. Have your client track their water too.
Your clients should build some strength training into their everyday training. You should show them some flexibility moves with the TRX, battle ropes, kettlebells and do some things for balance as well the upper arm workout. They should mix the moves up every week.