Plutarch Demosthenes 6

[1] However this may be, when Demosthenes came of age he began to bring suits against his guardians and to write speeches attacking them. They devised many evasions and new trials, but Demosthenes, after practising himself in these exercises, as Thucydides says, ¹ not without toil and danger, won his cause, although he was able to recover not even a small fraction of his patrimony. However, he acquired sufficient practice and confidence in speaking, and got a taste of the distinction and power that go with forensic contests, and therefore essayed to come forward and engage in public matters.

[2] And just as Laomedon the Orchomenian—so we are told—practised long-distance running by the advice of his physicians, to ward off some disease of the spleen, and then, after restoring his health in this way, entered the great games and became one of the best runners of the long course, so Demosthenes, after applying himself to oratory in the first place for the sake of recovering his private property, by this means acquired ability and power in speaking, and at last in public business, as it were in the great games, won the first place among the citizens who strove with one another on the bema.

[3]
  And yet when he first addressed the people he was interrupted by their clamours and laughed at for his inexperience, since his discourse seemed to them confused by long periods and too harshly and immoderately tortured by formal arguments. He had also, as it would appear, a certain weakness of voice and indistinctness of speech and shortness of breath which disturbed the sense of what he said by disjoining his sentences.

[4] And finally, when he had forsaken the assembly and was wandering about dejectedly in the Piraeus, Eunomus the Thriasian, who was already a very old man, caught sight of him and upbraided him because, although he had a style of speaking which was most like that of Pericles, he was throwing himself away out of weakness and lack of courage, neither facing the multitude with boldness, nor preparing his body for these forensic contests, but suffering it to wither away in slothful neglect.