The off piste at Cwmcarn starts from the uplift drop off, but the trails drop down the south side of the hill, rather than the north side, from the bottom you can ride along the canal path and up forest drive back to the uplift, but it’s not that quick*, and you would lose a run each time.
The other option is straight up darren road, https://strava.app.link/MJBsHlaNzgb
Edit.
*2.4 miles from the bottom of Wacko Jacko back to the car park, the canal path is currently closed, so you have to ride along the road through Risca, that was by far the scariest part of last weekends ride, pavements are to narrow to ride on and the road is lethal
https://strava.app.link/m6XDTgKNzgb