No, Janeway was also confirmed to have at least one prior command. In "Revulsion," at Tuvok's promotion ceremony, she said, "The first time I met Tuvok he dressed me down in front of three Starfleet admirals for failing to observe proper tactical procedures during my first command."
However, this does not need to refer to the command of a starship. Janeway could have been commanding a research team for all we know - an assignment comparable to what Acting Ensign Wesley Crusher did in "Pen Pals".
Indeed, it would make more sense for Lieutenant Tuvok to "dress down" Ensign Janeway than Commander Janeway. And more sense for Ensign Janeway to command a research team (possibly on a planetside assignment where tactical considerations theoretically existed, although only in the mind of a somewhat paranoid Vulcan Security specialist hothead) than a starship.
On the issue of how Kirk could have gotten the "prestigious" command of the Enterprise
without prior experience, it should probably be mentioned that a good way to gain prestige is to survive an unsurvivable mission. Quite possibly, Starfleet had chosen to expend the Enterprise
, along with a crew of expendables, in the long shot mission of probing out of the galaxy, so that more prestigious ships, commanders and crews could safely follow.
Assigning young Commander Kirk to be the CO of the forlorn hope through the mysterious barrier would serve two practical purposes. First, the likely failure would not deprive Starfleet of an experienced officer. Second, the possible success might see Kirk surveying space outside our galaxy for a long time, with youth a definite advantage for maximizing the time spent on station and the data gathered before the ship for a second time attempted the risky piercing of the barrier.
Were Starfleet to think along these lines, it might well pick the somewhat promising XO of a humble destroyer for this assignment, counting his daredevil attitude, lack of social commitments, and his recent loss of a Starfleet father as advantages over steadfast, mature family men who have nothing to prove.