The problem here is that you are probably running out of Trackpoints, (Waypoints are single locations), you have about 10,000 Trackpoints (only 500 Waypoints), they are autoimatically generated as you travel and when the track log is full, your unit will either stop adding new trackpoints or delete early trackpoints in order to store new ones.
From the Main Menu select Tracks, there will be a Track log percentage figure showing how full the log is, now select Setup, the first item is 'Wrap when full' which I guess you has selected, below that is 'Record Method', and 'Interval', this should fix your problem, you can tell the GPS how often it should store a new Trackpoint, you have Auto, Time and Distance. If you select Distance and 0.05 Km, your GPS will store a new Trackpoint every 50 Metres, so after 200km you should have 4000 Trackpoints (about 40% full)
