The things that stops me from doing planks longer than I can are my shoulders and wrists, depending on the position of the plank. I can do a minute, maybe a little bit more if I'm rested.
My shoulders and neck must be really weak, as they always hurt when doing any upper body stuff, even when the muscle I'm working is really in my arms. I can barely do more than 5-10 front or lateral raises, even with 5 lb. weights. It all just starts spasming.
However, after 8 months of boot camp, I can do "regular" push ups, although, sometimes when I'm on my 3d set of 18 I have to get into the girlie position. But before, I couldn't do more than 3.
That class sounds perfect, Catrin. And exercising in the early AM usually means you stick with it. You're an early riser, so it shouldn't be that hard.