Why would Two-Bit think Dally, Johnny and Pony were heroes all along, even before they saved the kids?

Like all the boys, Two-Bit does not have the best home life.  Because the boys lack a sturdy family foundation, they rely on each other to get through the trials of life.  Ponyboy talks about the importance of friendship, of allowing friends to sleep on couches, of being able to fight with friends but still call on them when needed.  In Two-Bit's eyes, Dally, Johnny, and Pony all stuck together.  They helped each other out when needed; they were loyal and behaved selflessly.  Their adherence to the code of friendship makes them heroes.