instructions are vague like "Go northeast".
That's the only instruction you really need for most audaxes - after a few hours [or 24hrs if it's a 600], just turn round and head southwest to get home.
Sarcasm aside, I've not used the 510, only the Etrex Vista since 2007, and like drossall am perfectly happy to follow a benign track [and yes.....until you forget to look at your unit!].
One thing I would emphasize, is the whole self-reliance thing with audax - particularly when you start to get into longer distances. I'm always amazed when I see riders still using a route sheet. But a laminated route sheet with lots of preparation before you set off [ie being aware of towns, shops etc] is still probably a pretty reliable and bomb-proof way to navigate.
Having said that, I don't always carry a routesheet as backup......and I should, but always carry a spare GPS unit loaded with a duplicate route.
I'd really encourage developing the habit of carrying a routesheet as back up.
Hope the ride goes well.