Don't let anyone put you off. I've trail ridden the early GSs all over Northumberland, North Yorkshire, the Peak District, the Alps and the Pyrenees. If you're sensible and never go anywhere you can't turn the bike around you'll be fine. And never, ever go down anything that you might not get back up if you do have to turn around.
You must do the Kielder Trail. It's gravel and cars can use it. It costs a pound I think. It goes from Kielder village, at the end of the reservoir, NW to the A68. You must also do Plenmeller Common, W of Hexham, and as stated above, the track heading north out of Blanchland to Slaley. There's two branches at the start and a bit of sand in the middle for which you'll just have to keep on the throttle and lean back (keep the front wheel light). There's also a lovely track that heads SW over the moors to The Tan Hill Inn, the highest pub in England. It's never warm up there and the fire in the bar has been kept alight for at least the last 100 years.
My brother rides with the TRF up there on a KTM 450. You wouldn't want to go with them on your bike and to be fair they probably wouldn't let you. I don't know what the big trail bike scene is like up there at the moment. When I lived up there and started trail riding a GS (1984) I think I might have been the only big trail bike for miles. There's plenty to do as long as you're sensible. There's good trails up around Alston too.
Make sure you've got crash bars and Continental TKC80 tyres, which you can let down to around 25psi to get a bit more grip. They're great on the road, even though they're knobbly but limit you to 100mph I think.
For maps you can do everything above on your phone with Google Maps. I've also got an old iPhone SE with all of the UK1:50,000 Ordnance Survey maps downloaded to it (that took 4 days!). They cost about £70 from Memory Maps a few years ago. Look for white tracks with the red dots on them. It sits in an Ultimate Addons waterproof case plugged in to a USB socket next to the speedo.
Steve.