Portage is an old voyageur term left over from the fur trade days. It is defined as "the act of carrying a canoe and packs overland between two bodies of water or to avoid river obstacles such as rapids and waterfalls." The trail used to carry canoe and gear is also known a portage. Portages are measured in "rods"--a rod is 16.5 feet or 5 meters. There are 320 rods to a mile.
On calm water with no headwind, two paddlers can usually travel two or three miles per hour by canoe, but the variable that makes it difficult to predict time and distance is the portage.
Unloading, carrying, and reloading your canoe and gear on portages takes considerable time..
Just knowing the length of a portage doesn't give you all the facts to estimate the time required to make a portage. There are too many other unknown factors, such as how long it will take to find and make a landing, the condition of the trail, changes in elevation. Some portages are nearly impassible because of muddy conditions, excessive tree roots, downfall trees, and landslides with loose rocks..
My advice is always allow an extra day or two, and bring a good book to read for those days that you find yourself wind bound.