Places to stop and walk dogs A40/A48/M4

I don't post often I know, but I do read a lot on the forum, honest

I and a few friends are coming over to the Forest of Dean next week from Ireland, for a sled dog rally, getting the ferry into Fishguard and wanted to stop to let the dogs stretch their legs. We want to get to our destination before dark, so that they can have a really good walk there, so we don't want to go too far off our route, and it doesn't need to be a fantastic woods or anything, just some grass, maybe some trees, and not right next to the road.

Can anyone recommend anywhere?

I stopped once at a place where there was a little bit of grass and a few trees, I think it was Llanelli Crosshands, by a Travel Lodge and a Lidl if I remember right, and I think there is a McDonalds the other side of the road, does this sound familiar to anyone? We'd quite like somewhere a bit nearer to Fishguard really, as they will have been on the ferry for over 4 hours by that stage.

Thanks