Swansea is excellent for MTB.
We have sanctioned trails at Clyne on the west side of town and Kilvey on the east. Both can be connected by the cycle path along the seafront, but with official and unofficial trails there’s easily a full day’s worth of riding at each location. These are both within 3 miles of the city centre. Grading is red for the official Clyne trails, but unofficial stuff at both locations can be bigger and techier.
Gower for XC riding – obviously rideable from the city, is gorgeous and is a two day trip to ride fully. Or one long, tiring day.
I also occasionally ride to Briton Ferry – 75% off-road en-route, – good, unofficial trail/enduro style. For me it’s a 30 mile round trip and can be extended towards Masts or Afan.
There’s XC stuff north of the city, where the hills start getting big and steep. Again, this is rideable from town and mainly off-road to get there,
Troserch woods over towards Llanelli is a decent sanctioned trail. There are unofficial trails all over the place which are of varying quality.
Most trail centres are fairly close – FoD is about 2 hours, and that’s the furthest one we regularly ride.
Gravel and road riding are excellent as well, and surfing is decent as are plenty of other outdoor pursuits.