ztar, I too paddle my double by myself at times -- primarily when I take my youngest daughter out -- she thinks she's a princess and can just sit and enjoy the ride. Since she weighs next to nothing, I also use water for ballast -- in my case, with my daughter, I fill up 2 or 3, 4-litre milk containers with water and put them in the front hatch. Basically, I consider the times with my daughter to be a "workout paddle".
The furthest I've ever paddled by myself, with my daughter in the front was about 12 km. Last Tuesday, Wednesday, and Thursday, that all changed as I took my double for a longer solo trip. I paddled a total of 55 km from Barnett Marine Park to the top of Indian Arm (and up the Salmon River), and back.
Overall, it wasn't too bad. I had about twenty litres of water in the front hatch and loaded most of my gear in the front cockpit (it was the quickest that I've ever loaded my boat for a trip). I averaged 4.7 km/hr on the way up against a slight headwind -- I wasn't paddling hard, but I was paddling steady -- I was quite often paddling 6.5 km/hr and my average would probably have been around 5.5 but I stopped to take about 300 pictures on the way up. On the way back I had the wind behind me and managed to average 8 km/hr for most of the way -- in fact, I travelled the 9 km from Granite Falls to Thwaytes Landing in an hour and ten minutes!
When I got to the bottom of the Arm where it meets with Burrard Inlet, the wind kicked up quite strong and there were between 1 foot and 2 foot wind waves that I was taking pretty much broadside. It wasn't too bad but if the waves were any bigger I most likely would have changed course to head more directly into the wind -- mind you, I'd probably do the same in my single.
It's definitely more work than paddling a single, but it was really nice taking scads more gear than I ever could dream of carrying in my skin-on-frame. Having a folding camp chair was a real treat.
Here's a picture of my dash: