Shifting dietary make up can be a challenge and the tougher it is to change the more you need to make these adjustments. If you are running out of steam in 2 hours that’s a good sign that you are not very well fat adapted.
The first days and even weeks can be particularly tough if you are used to a high carb diet. Expect lots of hunger and cravings.
If you are prone to snack then have snacks available like nuts, plain yogurt, cheese and jerky.
I did a 7 hour ski tour yesterday with over 5000 vertical breaking trail. Had 2 eggs/cheese and 2 strips of bacon for breakfast. Ate a 200cal bar midway and salmon/and salad for dinner. Energy levels good all day. When you stop being carb reliant and are fat adapted you will be amazed at how little food it takes you to function.