obviously you'll be doing more than just narrating the action, so identify the main themes of the comic so you'll always know what to come back to when you start improvising. besides the plot, what was TLH actually ABOUT (i haven't read it in a while... love/loss/justice/compromise/duty/revenge... did it ask any difficult questions as a book, what did the comparison between the three 'heroes' Dent Gordon and Batman achieve?)
get to know the character motivations inside and out, since you'll need to expand their internal monologues, etc, and their dialogue will need to be more descriptive etc in a prose form (since dialogue is best when minimal in a comic, not necessarily so in prose).
Decide now what might not really work as prose, and don't be afraid to entirely change things. People talk about 'staying true' to the source material, but that shouldn't mean literally word-for-word, but staying true to the spirit and message of the story, while the actual plot can be edited and changed and definately extended.
good luck.