The Great Allegheny Passage Trail is perfect for you. I've done that one in all kinds of variations, many options.

The Erie Canal Trail is another one that comes to mind - that has a yearly big supported ride that I think is going on right now. I've only done segments of that one.

The Ohio to Erie trail from Cleveland to Cincinnati has fewer options, I've done much of it, and there is a bit of road riding but on mostly very low traffic roads.

20 miles a day might be ambitious for a 5 year old if she is pedaling her own bike vs. in a trailer or on a third wheel kinda bike that we used when my daughter was that age.

John P.