Dooku question

Star Wars: Episode III Revenge of the Sith (novelization) wrote:However, only one death was in his plan, and this dumb-show was becoming tiresome. Not to mention tiring. The dark power that served him went only so far, and he was, after all, not a young man.

What means "The dark power that served him went only so far" line? Is Tyranus' dark side power worsened his physical condition?

It means that Dooku’s Force abilities can’t entirely make up for the fact that he’s so old.

As I'm sure you're aware, all Force Users use the Force to augment their physical abilities in combat and improve their stamina. Dooku explicitly struggles to keep up in fights - his stamina isn't as good as those younger than him - (i.e. "he was, after all, not a young man"), so he uses the Force (as aforementioned) to compensate for his lack of physical ability (e.g. "the weight of his years dropped by"). However, it can only compensate up to a certain point - it is limited (i.e. "the dark power that served him went only so far"), and thus Dooku is struggling to keep up in the duel, so he resolves to finish it quickly. That's essentially what your quote is saying - Dooku's augmentation only goes so far as to help him deal with his age impacting his stamina.