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Ride the woods till you get tired. Catch your breath. Repeat. What's the experience level, on a scale where 0 is 'still nervous to stand'?
Not sure what's close to you, but the wider trails off the parking lot at Wrentham are pretty beginner friendly. The short loop at Freetown is pretty smooth too, with the exception of the swamp portion but it's pretty short. Good place to turn laps.
Riding with other people lets you redefine what is possible. Also lets someone show you trails, and pick stuff that's good for you. But can have the side effect of feeling like you have to keep moving. I think the ideal is a little of both. Sometimes ride with people. Then go back on your own and try stuff again for as many times until you get bored. I find solo riding better to push my endurance because I can choose when my breaks are, and keep them as short as I like.