In reply to another_alex:
Sounds like a typical example of poor diet and dehydration. You've possibly depleted both; muscle glycogen and blood glycogen stores during your walk.
Muscle glycogen is primarily required for muscle exertion and blood glycogen for mental function. However, each can be used for the other through complicated metabolic processes.
It sounds like, having exhausted muscle glycogen stores, that your body has converted blood glycogen for energy - leaving you feeling physically and mentally drained. The danger of this is, if your bodies glycogen stores are depleted, your body looks for alternate sources of energy through gluconeogenesis which could include the breakdown of muscle mass.
You could try consuming a proper meal the night before consisting of good quality carbohydrate sources for your body to stockpile glycogen for your morning activities. Then after the walk you'd want to get some carbohydrates in you to replenish the depleted stores for normal daily function.
The key is to keep your blood glucose level relatively level as spikes can cause excess fat uptake. Blood glucose will generally rise and stay there for three hours after a meal, after this it starts depleting.
Good luck.
