Jumping uses stamina as well. Make sure you are just not out of, or too low on, stamina to use jump while running.

Staying well rested all the time helps. You can drop a camp fire in the open and sit next to it with x for basic comfort.

I have had one seed that the terrain generation was a bit extreme.. right next to the spawn in was a drop off to a creek that would easily trap you without food. It was kind of interesting dealing with that while making sure i always had good food etc. But as difficult as that was I was still able to manage it with the current system.

Stay well fed and well rested. It is your best defense against not being able to jump.
Waiting at the bottom of a wall / moat is common to allow your stamina to recover and get out.