I think the thing with GPS, especially if you're fairly new to the technology is to look at all the features and then find a list of problems. Very few people bother to try them out either, they assume they can just load up a route and go. Whatever software you plan your route on (RideWithGPS, Strava, CycleStreets, Komoot etc) will all have slightly different look and feel to them, most using Open Source mapping but all with a range of options like "prefer trails", "avoid main roads", "use most popular" and so on. Click on start point A and end point B and it'll direct you different routes depending on which option you choose.
When that file gets uploaded to the GPS, make sure the settings match. If you upload a .gpx or .tcx file to the unit and it was done using "most popular" but your GPS is set to "always use trails", the GPS will likely re-route a lot of it. As mentioned above what you're really uploading is a list of waypoints, not a complete track. This can be frustrating if you plan your road ride, upload it and head out and the unit is directing you off down every trail it can find. It is well worth doing a few trial runs with a new GPS on roads you know - upload a route, head out and see what it does. Try going deliberately off course - does it re-route you back to the last point you were on the route, does it try and "cut the corner" and join up later or does it bin the whole route off and direct you to the end point via its own creation?
At the top end, GPS cycling computers are like mini fitness centres, designed to connect to power, HR and cadence sensors, record all the metrics of a ride and use that to feed into your online training plans - they also do navigation but that is not really their primary function.
Middle of the road, you end up with the ones that are more designed for mapping/navigation and touring and travel rather than primarily performance metrics. However there are also significant differences between GPS for walking (which will show FPs, close up detail) and GPS for cycling (which will focus on roads and cycling trails like BWs and remove a lot of the memory-heavy close up detail).
You've bought is a walking / hiking specific one which will never be as good for long distance riding. On a walking one, you can get away with far fewer waypoints because you're travelling far less distance - however the map will be far more detailed, potentially even showing walls and pylons like a 1:25000 OS would. A long-distance trail walker might do 40 miles a day at a push whereas for a touring cyclist that is 3hrs riding time and you absolutely don't need that level of detail.
OP - ultimately I'd say you've bought the wrong GPS and while it can probably be persuaded to work without too many issues, you'd be far better off with something like a Garmin Edge Explore.
Re the comments about tech:
wearwell wrote:
Have to ask - is there any particular point in all this high tec navigation stuff as compared to straight paper maps, or is it just the novelty value? It looks and sounds so difficult!
Like any tool (and paper maps are just a tool as well) you have to know how to use them. You wouldn't walk into the hills with a map having never opened it before and assume you could read it when the need arose. So why assume that you can just turn a GPS on and follow a line? They both need some familiarisation.