Just picked up one of these watches second hand. What I have done so far for trail running new routes, or self-navigation trail races, is create a route in bikehike.co.uk set to Openstreetmaps, as it shows and lets you follow all footpaths. Then load it into a Garmin 705 and hold that in my hand. Much better mapping display than the watch obviously, but be nice to try the option of the watch too.
However with the 310xt watch it don't seem to let me load in a course from Bikehike. I realise you can create a course in Garmin Connect, but as far I can figure, Garmin Connect will display footpaths, but not let you automatically route along them.
Is there a way of taking the course you have created in Bikehike or other similar mapping website that lets you follow OS map foothpaths, and loading it into Garmin Connect, to then load onto the watch? Or another way to load your custom trail route into this watch?
I found a tutorial from dc rainmaker by doing a web search, but it's a few years old and uses something called Garmin Training Centre to do this job, which does not appear to exist any more.