I haven't been doing this to long so others may be able to give you better suggestions but the closest to Duluth I believe is Big Creek Park. I'm not a huge fan of it but it is pretty nice, very rocky, and unfortunately the freeride/dh stuff is closed
not that I was really tearing that up... The other two places I can come up with off the top of my head are harbins park in Dacula or blankets creek in Woodstock, or fort yargo all being probably around 35-60 min depending on where in Duluth you are ohh and the time of day...Atlanta traffic kills. Blankets would be my choice they have everything you could want from fast flowy to technical and hilly and SORBA Woodstock has done a great job with it. Last I heard there was construction or something at ft yargo I would have to check in to that. Harbins has plenty of trail and a fair variety of fast and some technical(enough to break my collar bone one time) ohh well but I love riding with folks and I do more of casual riding also, so holler when your down here!
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if you want to go ride.