28s will be fine as long as it isn't deep mud. I rode the Erie Canal trail on 25s and found then ok for surfaces ranging from tarmac, through loose gravel to sections of mud and rocks.
I'll admit that I've only ridden a few miles (in June) of the C&O Canal, but I was very glad I had 32s and wished I had 35s at several spots. I credited my incredible bike handling skills -) for staying upright through some of the muddy spots. 28s might suffice at the end of a dry summer or drought, though.