Delete the rides now. Garmins have tonnes of software bugs and one of them is corrupting the whole thing if there’s too many old rides in there.
Just connect to a PC and copy everything from the Activities folder to your PC if you want to keep the files, then delete them on the device, either from the folder or using the menus on the Garmin. You can keep the ‘totals’ that clocks up your miles etc.
If you want specific routes to follow you can load them onto the Garmin as routes. Garmin Connect and then syncing with Express in theory can do this though I found it rarely worked (I find Express a bit flaky anyway). There’s also Garmin Basecamp for the desktop which is quite powerful for doing routes.
Oh and tip for saving space – delete languages you don’t need from the ‘Text’ folder. If you only need English, delete everything in there. The languages take up a lot of space.
Also, backup the device profile and settings while you’re at it. Garmins do corrupt themselves whenever they feel like it and while you can reset it, it’s a pain setting it all up again. https://support.garmin.com/support/searchSupport/case.faces?caseId=%7B87d71a10-4ac5-11e2-e8a7-000000000000%7D