Remember Vong Krayt had to go to stasis after a bout with Cade Skywalker, Cade that was far from his prime too. Krayt had to go freezing just because of that duel.
Now let's consider Kenobi is a better duelist than Cade, and by that point in time, he was quite the superior one.
Interesting thing to note is that if Krayt was so taxed in that duel that he needed to freezing after that duel, fighting with Kenobi will be even more taxing. Cade is an aggresive fighter; Obi-Wan is on the defensive, meaning it will force Krayt to launch more of an offense, and offense is definitely more tiring.