Heh, that's true! I can be a 10x PBP veteran in no time!
Sadly my original suspicions about the commute were borne out in style on the return this evening, which I found pretty grim (too grim to imagine doing it for any length of time).
http://connect.garmin.com/activity/124095266
Considering that for years I happily rode out on the A90 towards Fife and back, this is saying something... it might be OK with a (two wheel) trailer or the deaf vest, but neither of them is really a solution, so-
(It's also a crikey-inducing 70 minutes return, versus 50 minutes in. That's almost half again as long..!)
I then experimented with this path concept on the way home. The surface to Auchendinny (that is to say, this section here) was unsurfaced but not muddy to speak of - at least it gave me no trouble on a Gatorskin, it was literally muddy I suppose.
The building site was rather confusing, because I wanted to exit to Auchendinny up the road I half-remembered, which now seems to be part of a building site (although they're clearly maintaining access along the path using fences, so a bit puzzling). Since there was nobody there I just went through the site, but naturally rather disappointing.
It's a fair winch up to the turn-off for Roslin, although it doesn't really take that much time. Round by the landfill was deserted and from Roslin to Loanhead is actually an extremely fast and well surfaced ride. I passed two anti-disabled barriers but they are of the canal-style staggered type, so didn't require me to slow down.
Loanhead to Straiton by that time was pretty dead, although in rush hour it might be slightly grim as you've to climb slightly at the end with very little width for anyone to overtake. It does make it very easy to negotiate the roundabout, which was a nice bonus. And then in to town as expected for a total of 40 minutes (moving time - plus 5 minutes being lost). So yeah, it's a third longer despite being just 9/8ths of the distance...
http://connect.garmin.com/activity/124095649
(Here you can see that I'm dying of hunger and wind exhaustion as much as a reflection of the hillyness / roughness of the path).