Scientific research has identified lactate threshold (LT) and maximal exercise performance as being very important contributors to endurance performance. A well-designed endurance training program can lead to improvement in both LT and maximal exercise performance within a single season and from season to season.
This article briefly describes how individuals can have the same training program, but have large fluctuations in the adaptation responses due to genetic and life style factors.
In this video from the NSCA's 2014 National Conference, Brad Schoenfeld and Alan Aragon discuss the evidence behind nutrient timing and exercise, focusing specifically on protein, amino acids and carbohydrate.