Shaun, I use a ski, k1, and a Mohican which is set up with a tiller and a full footboard.
I don't think that whether you push from your heels or your forefoot makes much difference "at marathon pace" because the foot pressure at that pace isn't nearly what it is in a 500/1000 m sprint.
Also, In a K1 or Mohican, you don't have the seat bucket under your thighs, and, in those boats, it's a bit more important to have a good solid full foot board contact that is not encumbered by moving pedals. On the other hand, in a ski with a regular hump under your knees, you can't get that nearly straightened 'on-side' knee through which you transfer force to the foot. Some of that force gets transferred to the backside of the knee hump as a result, as well as to both pedals.
As far as tiller vs pedals, I prefer a tiller on flatwater but only if I am in a boat like a K1 or the Mohican, because there is more area of contact on a full footboard. I have used the Mohican with tiller in waves, and while it works ok, it's not nearly as good as pedals. Think of how much rudder movement you use (left-right-left) while surfing and weaving from bump to bump with your ski/pedal setup and how quickly you can do that. That's what's so much more difficult to do with a tiller - you just can't input to the rudder nearly as quickly.
On a slightly different note, I do see tremendous value in training with a K1 on flatwater. In the K1, provided it's tippier than your ski, you are forced to learn to apply power to the blade while you have fewer points of contact between you and the boat, i.e. no seat back, no knee hump, and are oftentimes in a precarious position balance-wise. This allows you to better develop your balance, your torso strength/stability, your posture - but only if you challenge yourself with it. You can't hide in a tippy k1.