There is no way to save the end point of a worout on a route as the start point for the next workou. One has to start from the beginning. A workout is arround 60 - 90 minutes. Starting a longer route (e.g. on Watopia) I am finished before I can finish the route. It would be a nice feature if one cane store this point as starting point for the next workout. So one can use the longer distances for training purposes.
I aked Zwift and they agree, that this should be a good feature but I have to ask the community about this.