The Southern Upland Way is coast-to-coast trail that traverses Southern Scotland. It starts at the picturesque town of Portpatrick on Scotland's west coast, goes through the Galloway hills and the rougher Borders region, to finally end at the small village of Cockburnspath on the east coast. The SUW is rated at a moderate difficulty. Although the terrain is typically mild, the unpredictable weather can change that at a moment's notice. In particular, navigation can become quite difficult in high wind and rain.
The terrain on this trail is generally mild. There are many hilly regions which with significant elevation, but these are still relatively moderate compared to more mountainous trails. Additionally, there are long stretches of roads and flatter areas which reduce the overall terrain difficulty.
The SUW passes through many smaller towns and villages, so it is possible to generally rely on those for obtaining clean drinking water (from shops, public restrooms, etc). While on the trail, there are numerous streams (called burns in Scotland), where you can filter water. However, since there is a lot of animal waste through farmland, it may be prudent to avoid filtering in those parts.
There are enough towns along the trail so that you could potentially only need to carry a maximum of 2-3 days worth of food at any time. Accessing backpacking equipment will be much harder, as there are only a few larger towns that have shops selling outdoor gear - Stranraer towards the beginning, and Galashiels towards the end.
On the plus side, the temperature range while in season is very pleasant for hiking, at a range of about 40F lows to 60F highs. The nights will be chilly, but the days will be pleasant, and you won't be sweating or needing to drink as much water. However, unpredictable strong winds and rain can make hiking much more difficult, especially in featureless hilly regions.
Generally, the trail has excellent waymarking on signposts (blazes need to be on posts rather than trees or rocks, which are less permanent and fewer in number). Given that the trail was established in 1984, there are still some portions with signposts that are over 30 years old. These will be in worse condition and harder to spot. In some cases, the signposting is ambiguous, and other times it may be very hard to navigate in low visibility conditions. It is highly recommended to bring some form of detailed maps or GPS functionality as backup.
The primary hazard on the Southern Upland Way is simply the unpredictable weather. Scotland is a very windy region, and combined with rain it can become quite hazardous to traverse the hills. Other less severe hazards include midges and ticks, but the wildlife is not typically dangerous.
There are very few people who attempt to thru-hike the SUW, and even in season there are probably only a few people per week who start out the hike in Portpatrick. In this sense, it is quite an isolated trail. However, there are more occasional day hikers, as well as locals when passing through towns and villages along the way.
Scotland is a small enough country that even here in the countryside, you will usually be 1 or 2 days away (by walking) from a large enough town that includes bus transportation. Also, at most points along the route you will be able to obtain cell reception, and could potentially arrange a taxi or shuttle service to pick you up at the nearest road in case of an emergency. There is ample transportation from the starting point in Portpatrick, as well as at the ending town of Cockburnspath.
Many hikers from the UK will arrange nightly accommodation in advance and then have their gear transported to each night's stay. Since the towns are somewhat spread out, this does require some creative planning, which some companies can arrange for you (since B&Bs may need to pick up hikers and bring them off trail for the night). For those who carry their sleeping gear, you can camp just about anywhere thanks to Scotland's progressive outdoor code, or you can use the several bothies (shelters) along the way.