I am a GB age group athlete have used this training plan for my last 3 marathons and have twice run sub-3hrs using it in addition to my triathlon training, you should be comfortable running 10-15km before embarking on this plan.
This is a 16 week marathon training plan for triathletes to be supplemented with additional cross-training activities such as strength workouts, turbos, bike and swim sessions - although it could also be used as a basic stand alone marathon training plan if required.
Regular sports massages are encouraged during the training plan, I usually book a massage every 3 weeks during marathon training.
Make sure you take sufficient nutrition and hydration with you for the long runs, the 4 x 20+ milers are the critical workouts I have found, i usually get straight in to an ice bath following these 20 mile runs, brrr..........!