This is one of the better-studied use cases, and the short answer is yes, with the strongest evidence pointing to the Paraxanthine, with L-theanine playing a supporting role.
In a 2024 trial during a 10-kilometer run, Paraxanthine improved mental sharpness and reduced exercise-related mental fatigue, with some measures favoring Paraxanthine over caffeine.
The mechanism makes sense. Paraxanthine, like caffeine, takes the edge off perceived effort. It also supports better blood flow through a separate pathway, and gives a mild boost to motivation chemicals like dopamine. For a runner trying to hold a hard pace for an hour, those small advantages add up.
L-theanine's main contribution to long runs is mental control: less pre-race anxiety, more steady focus through the middle miles where mental discipline tends to fall apart. Combined with Paraxanthine, it produces sharper attention with fewer of the jitters.
So tuck a pouch about 30 to 60 minutes before the run. Hydrate well. Get at it.