When I walked last year in late September I too only had the light fleece layer. By then the days are getting shorter, and so, yes, the mornings are coolish but nothing that made me wish I'd got dressed in warmer clothes. I carry a lightweight windproof jacket (it is also waterproof) as extra insulation and there were a couple of times that I used that when I stopped for a rest, but that was easier than putting on the fleece. I only walk with a long sleeved shirt and shorts, and although I have a fleece I never used it to walk in, I was warm enough once I started, with the shirt (and shorts). Mind you, I don't feel the cold - factor that into your calculations - how YOU feel the cold.
Cheers, janet