Valine is one of three Essential Amino Acids (EAA) that are considered Branched-chain Amino Acids (BCAA). Valine, like the other BCAAs, promotes Muscle Growth and Muscular Recover. According to PubMed, Valine can also be used to treat metabolic and liver diseases in clinical settings. 
Leucine, like all BCAAs, are different from the other EAAs because they are metabolized in your muscles and not your liver, like the other EAAs.
Your body gets tired when you train partly due to increased Serotonin (5-HT) levels caused by an exercise-induced release of that neurotransmitter.
The EAA Tryptophan is the Amino Acid that is most responsible for the creation of Serotonin in the brain.
BCAAs and especially Valine compete with Tryptophan for entry to the brain and studies illustrate that supplementing with BCAAs Pre Workout, especially Valine, can reduce the amount of Tryptophan that makes it to the brain and; therefore, reduce Serotonin production and the resulting fatigue.
It seems like Supplementing with a High Quality BCAA Supplement Pre Workout and Intra Workout could increase Muscular Endurance and help you achieve more during your longest workouts.
The fatigue caused by 5-HT is not limited to the body, but it also impacts the brain. When BCAAs, and especially Valine, supplementation results in reduced 5-HT in the brain it also reduces mental fatigue.
Supplementing with BCAAs Pre Workout, Intra Workout, and Throughout the Day could help keep you mentally and physically alert.