It covers up to Shrewsbury via HoW and the Marches line, and also as far as Gloucester via Lydney.
http://www.nationalrail.co.uk/times_fares/pre459c70a04000200159992e103b671.aspx
To get your moneys worth I would do the WAG set to at least Hereford, if not Shrewsbury. I would also try and fit in a few FGW HST moves as they won't be HST for long.
The OP has around 3 years before the HSTs go, but yes I would recommend enjoying the experience in case they don't get another chance to do so.
Small town in West Wales
https://www.google.co.uk/maps/@51.6973304,-4.9749711,12z
Nice place to spend a sunny June Saturday sitting on the banks overlooking the river, peaceful.
No problem, it's good value for the area covered, and I've used one several times!