I think physical fitness is of great importance, though. All of the Forsaken, Rand, Taim, Logain... are physically strong and fit. Even older Aes Sedai like Be'lal are described as showing no frailty and being powerful even when they walk. This makes sense, considering it's so tiring to channel - the power corrodes at your life force until you recover, and fatigues you quickly. I honestly don't think you can maintain Forsaken strength channeling if you become unfit or ill.

It's one of my theories on Aginor and Balthamel. Both were greatly weakened by their old bodies, and Balthamel was killed by a Nym and Aginor overdrew accidentally. Ordinarily they would be far more powerful.