That road is entirely ridable and really nice. The other side of Loch Katrine is a bike path, as well. But I think Inversnaid north along Lomond is the problem area? You could take the ferry across Inversnaid and take the A82 (I think there's a bike lane alone here as well?) until you get back on the trail.
Ah ok, no I don't think the bike path goes that far up on the A82, starts around Tarbert heading south from memory and it's not a nice road at all to ride on. It would be quicker but not at all pleasant.
That road is entirely ridable and really nice. The other side of Loch Katrine is a bike path, as well. But I think Inversnaid north along Lomond is the problem area? You could take the ferry across Inversnaid and take the A82 (I think there's a bike lane alone here as well?) until you get back on the trail.