Visiting a new dojo a couple of months ago, I got to point where I was about to pass out. (Maybe I should lose some weight....) The pace was so fast and intense it nearly did me in.
Returning the next week I resolved not to let this happen to me again and tried to figure out how I would survive.
In the end I fell back on the things that my teacher is always reminding me about, which were; a soft and relaxed upper body and breathing. By wasting no energy in "fighting" my uke and keeping my ukemi as soft and flowing as I could manage when I was uke, I finished the evening feeling quite good, in stark contrast to the week before.
It took a hard lesson to force me to learn to value of the fundementals, but do we really learn any other way?