I took my middle child for a run round the Pentlands on Friday (yes, the day with the sleet)...
We set off clockwise from Flotterstone, Glencorse Reservoir, Loganlea Reservoir, Harlaw Reservoir, then directly back to Glencorse via Maiden's Cleugh. The direction was chosen to reduce the gradient, leaving the steepest section for the downhill stretch.
This route is marked in green on the official map ("surfaced paths or tracks suitable for careful cycling"), and is indeed a tarmac road to Loganlea Reservoir. The track to Bavelaw was also not bad, bearing in mind that we were both on commuter bikes rather than mountain bikes, and from there to Harlaw is again a mix of tarmac and good paths.
However, the climb back up from Harlaw involved quite large steps, and the short descent to Glencorse was dreadful! We made it, walking a couple of sections, which had drops of 6" or more, but the rest was scree rather than surfaced track.
Perhaps most here knew this already, but I thought it worth having here in case anyone else ends up committed to more than they expected...
I'd do it again, but would return home from Harlaw via Balerno and the Water of Leith - it would make little difference to the day's mileage (~27) for us, but not really an option if you've left your car at Flotterstone!