Did this about 20 years ago with our boys (14 & 16 at the time). I believe we did the ft benton to kipp st park, taking 5 days.  That seems like a long way in five days but going down stream, maybe 5 mph for 6 hrs per day is what we did.  It is pretty much a float trip, not much in the way of rapids.

We had to bring our own water,  I think we had two jugs holding 10-12 gallons total.  I think a fill-up about a third of the way through.  Some nice scenery but also lots of passing cows out to pasture.  The most vivid memory were the fenced in campgrounds.  It was like we were the animals in a cage, for the enjoyment of the cows all around.