Maybe keep your kite higher. That makes it lot less strenous for the legs. My legs are fine. All things cosidered it is lighter for me that TT. But with TT i spent all the time jumping and now I am back practising turns
Yes, you use almost no muscles to foil - when you have learned you can ride "forever" without getting tired, IF you want that
Or you can put a big kite up, fly it low and powered while pushing upwind, and get a workout - or you can do tricks or ride waves so you get tired fast because of the moves themself, or the crashes, or the pure stoke ha haa
While learning, you can easily cramp up in no time, because of the tension trying to balance and control "just some of it for a little while", and after 20 minutes many (me included back when starting full on from day one) are toast, the very first days.
PS: I see no reason why super deep downwind is tiring Tone ! Going at max speed downwind can be, but you can go even deeper at a tad less speed, so super deep is easy IMO.
I meant super downwind with the kite low and powered. I can stand and go downwind with the kite at noon all day long but when I want to see how fast I can go off the wind or very deep and hiked over, it kills my legs.