The FBU has called a strike across eight days, starting from 9 August, over the age at which its members can draw their pensions.
The dispute centres around Government plans to increase the Normal Pension Age (NPA) for firemen from 55 to 60 by the end of 2015.
Dark told HR magazine the uniquely physical nature of the job means the new age is unrealistic.
"The Government's argument is based around people living longer," he said. "But it's not about that. It's about people being able to do the job. While there are some exceptionally capable people, the majority won't stay fit enough to do this."
While other physically demanding professions have more sedentary roles available for older workers, in the fire service those jobs "simply aren't there", according to Dark.
Earlier this month former fire minister Brandon Lewis accused the FBU of "disrupting a tax-payer funded service" with unnecessary strikes.
"The equivalent private pension pot would be worth over half a million pounds and require firefighters to contribute twice as much," he said.
But Dark denied it was an issue of comparing private and public sector pensions, stating that it was not appropriate to get involved in a "race to the bottom". He added that both Government and FBU figures showed that funds are there to cover the extra five years of pensions.
"It's not even a case of going to the Treasury and asking for more money," he said. "All it takes is a little re-jigging. I think the Government advisers we meet know we're right about this."