great plan re the pedal kayak - it is a rather different action & form of exercise from the things you think it should be like, such as walking/running, cycling and stepping.
And one of the great things is it is very low impact so long as you have the pedals adjusted correctly for your leg length = so that your knee remains slightly bent at full stretch.
I am not sure what hydrodynamics or other physics is at play but basically the faster you try to go, the greater the resistance becomes (or so it appears), so the rate of exercise seems to be self-regulating: you can only go at it so hard because, beyond a certain point, extra effort is not rewarded by extra speed - and you never get to a point where it becomes really high impact like you can on a bike, stepper, treadmill, weights.
What all this means is that (once you are conditioned to the exercise) you can keep going for hours and hours with a somewhat raised heart rate without overdoing it and without putting great stress on your joints. I can pedal my kayak for 6-8 hours without feeling that my body is stressed like I would if I had been out walking or cycling for the same amount of time. I still feel weary, but it is not a battered and bruised/aches and pains feeling. I am not yet at the stage where I want to give up other forms of exercise but I do think that I will be able to carry on Hobie kayaking for a lot longer than I will be able to go trail running and mountain biking.
For my part I find that pedalling my kayak exercises the muscles in my legs, butt and strangely, abs (in the area of solar plexus).
I also find that I can't carry the refrigerator on the load tray, so when I am out there for hours at a time I am definitely expending more calories than I am consuming
Good luck with it ! Enjoy !!