Hi, Scougs, it's me again.
If I recall correctly (it has been a few years), this section shouldn't offer any difficulties based on what you've said about yourself and your abilities; there might be a step or two "up" that's a bit larger than you might naturally want to make, but I don't remember anything that could be close to described as even an "easy" scramble. (btw, I've not done the section of the Fife coastal path you talk about, so I can't compare the two.)
There are areas along Loch Lomond that are rocky and have a lot of exposed tree roots. But nothing overtly difficult, other that just watching the terrain and paying attention to where your feet are going to land.
I think you'll probably be fine. There may be others who can chime in with more recent information than I have, but I can't imagine that this section will have changed too much during the intervening years.
The first time I did the WHW, my biggest challenge was getting firmly stuck in the mud a bit north of Inverarnan; I honestly thought I was going to have to wait for someone to come along and pull me out! (I eventually managed to pull myself free.) There was also a precipitous bit along the river in this area, but when I did it again two years later, the path had been rerouted to avoid both of these "dangers."