You'll be fine.
"your going on an 8 day trip from sand island to clay hills" what a commercial san juan guide said, when I told him I'd never run it before.
Either section is great if you don't have the time. If your not good at avoiding sand bars, you will be lot's better after the section above clay hills. Don't over pack, and avoid the center of the river below slickhorn, for the most part at least.
Fall can be nice, October is often storm season.
" I wish I were a headlight, on a North bound train..."