I think it's important to note that Michonne didn't originally set out to follow Rick's tracks. She purposefully went the other direction--perhaps to go it alone seeing as how it was just too painful to lose anyone else.
It's only AFTER she kills the entire herd/has nightmares about her past life/etc. that she realizes no one can make it alone anymore.
THEN she gets back to the road and follows the track she originally saw. We got to know a LOT about Michonne's inner workings this episode.