hm as of right now I can't tell you of any great way....

basically you need the avarage velocity.. to get that you need the distance... you could get the distance by doing a for loop and adding up the distance from point i and i+1... than you can get the average velocity... than you could put marks in some array how far every point is... than you could calculate how far you are at the time the user opens the form... than you first go to the point it needs to be
from there on you need to get the way from this point to the next one as a function which would be easy... well I guess you know what I mean and you could have just come up with it yourself...
I really can't think of any more elegant way right now...