This is a 7- 8 mile walk onto Mynydd Myddfai (Myddfai mountain), which briefly skirts the Usk Reservoir.
The Physicians' Well is signposted from various points around Myddfai (you'll pass two of them on Walk 2, at Cwmclyd and Cymnantybeudy), but it's quite hard to find. Although the well itself is little more than a trickle, it's a good destination point for exploring Mynydd Myddfai.
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The walk begins at the bottom of the village, where you turn left at Yr Arch on the road going east. Follow the road for a mile (passing the 13th century mill ruin on your right) up and then down until you get to a stone house (Sarnau). Look for the signpost directing you through the gate, then turn immediately left through another signposted gate, walking parallel to the fence on your right (this section can be muddy). Go through another gate and continue straight on. At the next gate you''ll pass onto common land, where the path becomes better defined. After about a ¼ mile, the path bears right uphill. Follow it straight up, and when it bears right again, leave it and continue going straight up on a less well-defined path. This path will work its way up around the left hand side of the mountain. When it levels out, you should be above the ravine to your left, and on a clear day you will see Pen Y Fan in the distance straight ahead of you, with Fan Brycheiniog to your right.
The path will soon meet another better defined one (along the ridge in Walk 2): there are two paths going left, take the further one heading downhill (it will veer right, down then up), then follow the sheep track off it heading towards the wood. The sheep track will take you along the left hand edge of a section of longer, thicker grass (this usually means a boggy area), before crossing it at the shortest, driest point and heading back downhill towards the wood, with Fan Brycheiniog straight ahead of you. As you near the wood, you should be able to pick out a wooden signpost (pointing to the Physicians' Well) about 75 metres to your right. When you find yourself between the signpost and the wood, leave the path and head straight for a small gate into the wood.
Once through the gate, ignore paths right and left and carry straight on for about 80 metres. At the clearing on your left you'll see a wooden seat, and just past it, a tin cup on a chain above the Physicians' Well. After a drink at the well or a rest on the seat, continue along the path, turning right at the signpost (which point back to the Physicians' Well), and then through the wood for around half a mile. Look out for a small red signpost on your right, marking a path straight downhill (you'll see Fan Brycheiniog ahead of you). Drop straight down (the path can get overgrown, so look out for brambles) catching glimpses of the Usk Reservoir through the trees. When the path meets a track at the bottom, turn right. You'll shortly reach a picnic table - a good place to stop for lunch or a snack.
When you cross over a footbridge near the picnic table, look out for a makeshift gate (which forms part of the fence) to your right. Go through the gate, and walk up the valley ahead of you, keeping the valley bottom to your left. Head towards a path bearing right uphill ahead in the distance. As you get nearer, you will see another path which heads more steeply - sharp right - uphill. Follow this path straight up.
As you climb, you'll have views of the Usk Reservoir behind you. When the path levels out, it hovers along a contour line and you'll see it curve around the next mountain ahead of you. Carry on until the path drops down to an old stone pit above a fence (where you cross the path taken on Walk 2). From here take the left hand path, which goes gently downhill towards a stile in the fence. Cross over the stile and head downhill, keeping to the left of the ridge. After a boggy section the path will follow a trench, passing a sign pointing back up to the Physicians' Well. The path continues down past a smaller signpost into bushes and trees. From here the path will bear right and head straight downhill (the path can become a shallow stream in wet weather - still walkable but muddy in places), with lovely views of Myddfai below. You'll go through a gate, before continuing onto the road at Sarnau, which leads back the way you came to Myddfai.