It's raining buckets as I type.
More heavy rain forecast for Friday and light rain forecast for Saturday morning.
It is mostly hard pack, not paved though if that's what you mean.
I rode this route 3 times, twice from Glasgow, once from Edinburgh.
You will get muddy, you probably will walk some bits, the cobbled bridge for sure.
I rode under the tunnel.
To me, the canal route all the way to Edinburgh is long, boring and tiring.
Also, expect to be slowed down at the narrow bits by lots of dog walkers and ramblers.
I would take the road