Save the route as a GPX file (Mapsource - Save As and select GPX before saving, ITN Converter - Save as GPX format). Save this to either the device internal memory or the SD card, into the \\Garmin\GPX folder (if there isn't one create one). Then load the route using the menus on the Nuvi - Tools > scroll down > My Data > Import Route from File. You then select the route by Tools > scroll down > Custom Routes and then select the route you want.
You can have only 10 routes in the Custom Routes menu but you can store as many as you like on the card or in the internal memory and load them as you need them.
Works like a charm, used on Euro tour and on plenty of rides in the UK