I've run from Wachesaw Landing on the Waccamaw River, all the way to Little River Inlet, and back. That's about 40 miles one way, or 80 for the round trip.
When I was a kid, I made another run about the same distance, going from Cherry Grove Beach, SC, to Lockwood's Folly Inlet, NC, along the ICW. And that was before the days of cellphones. I can't even imagine traveling that distance away from home without a cellphone now.
The miles seem to go by quicker when you're traveling on a developed stretch of river with landings, marinas, etc.....such as a stretch of the ICW. It seems to drag on forever when you're traveling a stretch of isolated, uninhabited river, such as in the Santee Delta.
Also, on the ICW, there's a marina every 10-15 miles, so, there really isn't an issue with running out of fuel.
But, for those runs where there are no marinas, my 16 foot Triton has 24 gallons of fuel on board. My Aluma-Jet will carry 22 gallons. With either boat, my cruise range is close to 100 miles.