Not long home from doing this :-) Really enjoyed it. The route isn't very well sign posted as parts but I was able to work it out. Worst parts are going through Airdrie and after Livingstone. The nicest part I would say is after Plains. The worst is the whole lot of it after Livingstone. From Airdrie to the other end of Livingstone you are either on Roads or paths that are in much better conditions than the roads are. After Livingstone there is about a mile of dirt tracks and a mix between roads, dirt tracks and poorly maintained paths. I eventually got fed up with the it and the lack of signage around Balerno and jumped on the A71 and made my way to Haymarket (stopping at Tynecastle to take a picture to add it to my collection of Football Stadiums).
Anyone thinking of doing it I would highly recommend it, can't say much about before Airdrie but between Airdrie and the other end of Livingstone it is great. Some really great views and well maintained paths.
You can see my route from where I joined the route to Haymarket at
http://connect.garmin.com/course/4380051 I've cut out the part before it as it was mostly roads unrelated when I was making my way there from the house. I've also kept in the A71 part in case anyone wishes to follow it. I would recommend joining the A71 at the set of traffic lights just before Kirknewton though and not following the sings up the hill.
Looking back at the route though I think I may have took the wrong direction at one point as I'm aware there are 2 routes, the main route (red) and a blue route. But I ended up following signs and this is the way I ended up going. I might do it again next month as I have a feeling where I went wrong and I'll re post if I do.
I've attached some pictures. I've they haven't worked let me know and I'll retry and upload them.
The picture of the lake is just after the big lake after Plains. You can't really see the path well but it's smooth Tarmac and no bumps/holes at all. I managed to get up to 30MPH a good number of times on a Mountain Bike on the straight. Usually I struggle to get to this speed on a downhill on a road! lol
I'm happy to answer any questions so fire away.