The human body is a very adaptable machine. In no time at all it can adapt to most any kind of condition it is put through. This is why exercise becomes easier over time. The body becomes more efficient at the particular exercise because it expects it and makes the required changes to perform the exercise.
Your body has gotten used to the 30 minute jog so when you do the jog, it regulates your blood oxigen levels, your heart rate, your muscle contractions, etc to suit the task. When you walk up a hill, your body is confronted with a task it is not prepared for so you fatigue much more quickly.
This is essential to keeping in shape. This is why it is good practice to switch things up every few weeks when you exercise regularly. Never let the body get used to a particular movement. Keep it confused all the time and you will see results much quicker.
Edited to add...
Some sources regarding workout plateaus:
Link 2 *See "Problem 4" and "problem 5"
Note: A simple Google search can provide you with any number of references to workout plateaus.