"the only way to gain managerial experience is to manage"

Not exactly, and if it means that every new party leader comes to the job equally clueless and has to learn by doing, then no. Most new leaders of the opposition have been shadow spokespeople or even past cabinet ministers, they've long been associates of the previous leaders, they've seen how it's done from closeup and they've led subordinately themselves. Corbyn hadn't done any of that. That's an explanation for his difficulties, not an excuse. Maybe he should have learned faster.