The travel leg of the journey refers to the transportation portion of the trip. This includes details such as the mode of transportation, departure and arrival times, and any necessary travel arrangements.
That would depend on the context. If you are starting on a trip, that would be "a journey" If you are partway through a trip, you would be on "a leg of a journey" For example, if you were driving from New York to Los Angeles, that would be "a journey". Along the way, the portion of the trip between say Chicago and Kansas City might be "a leg of the journey"
A "leg"
The 2009 Dodge Journey has 40.8 in. of front leg room.
The 2011 Dodge Journey has 40.8 in. of front leg room.
The 2013 Dodge Journey has 40.8 in. of front leg room.
The 2014 Dodge Journey has 40.8 in. of front leg room.
The 2013 Dodge Journey has 36.1 in. of rear leg room.
The 2012 Dodge Journey has 40.8 in. of front leg room.
The 2014 Dodge Journey has 36.1 in. of rear leg room.
The 2010 Dodge Journey has 40.8 in. of front leg room.
The 2011 Dodge Journey has 33.6 in. of rear leg room.
The 2010 Dodge Journey has 33.6 in. of rear leg room.