Quote 19
We seemed to be nothing but children playing against heroic men (7.79).
…Yet the battles Gene fights, against his own fear and resentment, are far from child's play.
Quote 20
That night I made for the first time the kind of journey which later became the monotonous routine of my life; traveling through unknown countryside from one unknown settlement to another. The next year thus became the passing dominant activity, or rather passivity, of my army career, not fighting, not marching, but this kind of nighttime ricochet; for as it turned out I never got to the war (10.1).
The idea of Gene constantly traveling but never fighting is an apt one – he's essentially looking for the enemy soldiers but is unable to find them. So he picks something, or someone, to be his enemy. Like Finny.
Quote 21
To keep silent about this amazing happening deepened this shock for me. It made Finny seem too unusual for—not friendship, but too unusual for rivalry. And there were few relationships among us at Devon not based on rivalry (3.63).
This may have something to do with Gene's desire to become Phineas. Since he cannot compete with the boy, he doesn't know any other way to relate to him. He can't exist in relation to Finny, so he becomes one with Finny.