Uploading file to create a course, only uses a small part of the overall route

Could you post the original GPX file that you are trying to import? This effect usually happens when the GPX file contains multiple sections, instead of a single track. Some tools indeed do that. Which tool did you use for creating the GPX file?

Anyway, if you are trying to create a track for a multi-days trip, I discourage you from creating just a single GPX file. The number of track points per file is limited to 10,000 (and up to 500 way points), so first of all the file will be optimized during the import, resulting in a far too low resolution, and putting unnecessary load on your device, potentially slowing it down or even crashing. You better create a series of smaller sections of say ~50-100 km, and load them into the activity as needed. 

I am not able to attach the file, Garmin doesn't allow GPX or TXC files to be shared on here, which is again, bizzare. I have tried downloading it from several sources, but it doesn't work from any of them. Try downloading any eurovelo route from https://en.eurovelo.com/. And since I am not able to upload the route at all, I am not able to break it into smaller sections as you suggest. It seems absolutely bizarre that garmin isn't able to handle a route with multiple sections.

WileyandFabio said:

Garmin doesn't allow GPX or TXC files to be shared on here

Zip the file, then in a reply post Insert >> Image/video/file >> click Upload >> browse for file >> OK.

Also, GPX files imported into Garmin Connect ignore multiple track segments (trkSeg). In a similar thread, using Garmin Basecamp might allow you to make the file useable - GPX course download shows a shorter course than real course