During exertion efforts, your body uses glycogen stored in your muscles and liver for fuel. Your stored glycogen is sort of like fuel in your gas tank. As it's used, it's replaced by the body, using food that's ingested and absorbed into your bloodstream, and from fat and muscle fiber, which the body metabolizes. This glycogen replacement can't keep up with the need during high-intensity sustained efforts, which is why athletes ingest sugar during these efforts. If the intensity and duration is long, approaching 2 hours in my case, glycogen depletion can occur. This is what runners call "hitting the wall", and bikers call "bonking". When this happens, your body shuts down. It's a dramatic and sudden event. I've experienced this quite a few times before, on long bike rides. It doesn't happen from working in the yard or similar low-intensity efforts, because the body can replenish fast enough for this.
Before marathons or bike rides of 100 miles or more, atheletes eat lots of carbs, an event often called "carbo loading". This is a ritual that's of minimal value, but fun; eating a ton of pasta the night before the event, for example. The idea is to top off your glycogen storage tank. Thinking about this brings me back to my heavy biking days, when 3 and 4-hour bike rides on weekends were the norm. Again, you won't run out of glycogen during low-intensity exercise; it only happens during sustained efforts, commonly at the 20-mile mark for a marathoner.
