The excerpt that best confirms the theme of "the most difficult trials come with great reward" is the first one:
"If in that snowy backyard the driver of the black Buick had cut off our heads, Mikey’s and mine, I would have died happy, for nothing has required so much of me since as being chased all over Pittsburgh in the middle of winter—running terrified, exhausted—by this sainted, skinny, furious redheaded man who wished to have a word with us."
In this passage, the speaker reflects on being chased in a challenging situation but expresses a sense of fulfillment and happiness despite the fear and exhaustion experienced during that difficult trial. The notion that such trials are rewarding, in this case, is captured in the speaker’s willingness to embrace the experience as profoundly significant and even joyous.