I assume you saw this on the Cycling section:
Cycling
Many sections of the Way are excellent for mountain biking with a wide range of tracks from forest roads to singletrack. However other areas are unsurfaced track over grassy hillsides. Due to the prevailing weather conditions in southern Scotland and the fact that much of the area is peat bog these areas often become so muddy that it is not possible to cycle them. This is very weather dependant so areas that might be easy to ride in the winter when the ground is frozen are impassable over the summer and autumn.
We are currently working on improving the Way for Cyclists and also producing a guide to riding the Southern Upland Way with advice on which areas can be ridden and alternative routes for areas that are impassable. For more information please contact one of the Rangers.
Looks like it's not one for just yet, but certainly one to keep an eye on perhaps?
Were you thinking a one-hit-wonder, or a multi-day ride?