Calm conditions and no currents will be a big help both in predicting how much water you can cover and how easy it will be to do. Under those conditions I would expect to cover about 4mph for a 10 mile round trip, but I'm a notorious "fast paddler."

For a "moderately fit" paddler 3 hours would seem in the ballpark for a 10 mile round trip. That assumes that you take only short rest stops every couple of miles and that you are not travelling in a group. With a group of three or more people I'd allow 4 hours.

That brings up another point. A 5 mile crossing by a single paddler is inherently risky. All it takes is an inattentive power boater, an attack of food poisoning, or a muscle strain to leave you in a precarious position.