Sorry - it doesn't convert it into a route, it adds a new route based on it.
It will work better for a 200 if you split it up into segments first.
The way it works is to take each segment, reduce it (but only internally, in its own memory - it leaves the original track there) to 10 points, then asks google for a route through those 10 points.
The reason it has to reduce each segment to 10 is because 8 is the limit for number of intermediate waypoints, so that, plus start and end, is 10. But it does that for each segment of the route, not the whole route itself, so obviously the more segments you have the more fine grained and potentially accurate it is.
Reducing it to 500 first should have no (or insignificant) effect because it reduces each segment to 10 internally anyway.
It's by no means perfect - you will almost certainly have to fiddle around with it a bit, like add, remove and move the points on the route (but you should be able to do that now) however it should at least have a stab at pointing out junctions.
edit: it's not a bad idea to get into the hang of splitting into segments anyway - as a way of grouping your tracks into logical blocks. Certainly split it at least at controls, but further, any section that could potentially be re-routed, i.e. any section for which there's a potential choice of different ways to go, is better being its own segment.