I exercise at length so electrify the fence
To do something at length means to do something for a long period of time. However Lupe is talking about exercise, so he is saying he trains over both long distances and for a long time. Long distance running has served as a metaphor previously. Lupe has often been treated differently to his peers because of both his subject matter (alluded to in the imagined diss by Earl earlier in this track) and lyrical skill.
It should be noted there are many meanings for exercise including performing an activity (which could be mental rather than physical) to train and develop.
The electric fence represents a hazardous barrier either meant to keep Lupe out or to keep others away from him. “Electrifying” is another way to describe a thrilling experience. Perhaps for Lupe breaking through barriers and challenging himself is his own secret excitement, which sets up the next line.
To help improve the meaning of these lyrics, visit "Peace of Paper/Cup of Jayzus" by Lupe Fiasco Lyrics and leave a comment on the lyrics box