We have received a similar question in the past, but for review, I answer. Three speedy trial rules exist in Pennsylvania in criminal law: the statute of limitations, Rule 600, and pre-arrest delay. In your scenario, none apply because YOU skipped town. The prosecution should not get punished due to YOUR actions. Therefore, none of the three rules apply to you and the district attorney can still prosecute you.
In the previous questions and subsequent answers we talked about witnesses appearing or not, but in your case I think the analysis is a bit simpler. The prosecution needs to show that the weed was weed; that the mary jane contained htc (delta-90tetrahydrocannabinol). I am not sure how they show that twenty year marijuana wasn't tobacco.
They can try to prosecute you, but I am not sure if they can get a conviction.