I just can't see this being related to being in shape or not at all. I never do any exercise and smoke and drink as much as possible, I'm fine shredding all day and days in a row.
However, stance and technique can play a simply massive role in these things. I had the same problem as you when I first started, and I would hazard a guess your issue is the exact same. When I was on my first couple days all I knew was falling leaf and snow snowploughing. I would pick up speed and break hard on my heel side and then go the other way in switch and then snowplough again. Thats all I would do and it burns the crap out of that exact muscle you are talking about. The second I learned to link turns that pain was gone forever. I used turns to control speed instead of heel-side breaking.
If you aren't linking turns yet and this sounds like what your doing then I would say get a lesson, you sound like your in good shape, once you get your technique down you should be able to shred all day without a problem.