If you’re travelling from Bristol and don’t want to be on a train all day. or forking out for a huge fare, then how about this fairly close to you:
Bristol – Taunton or Bridgwater, then a 8.5 mile ride on road to Quantock hills. So you have to factor in 17+ road miles, which is a lot. You could take it easy and it would be OK, and you’d have loads of time to get home.
Bristol – Ivybridge (Dartmoor) and it’s right on the moors, with a few off road options straight from the station.
Bristol – Newport – Abergavenney for Black mountains, lots of great routes nearby
Bristol – Cardiff – Crosskeys If you want to go trail centre and go and do Cwmcarn. Good winter option perhaps?