I don’t jog much. In the past I’ve only gone up to about 2 miles. At that point it seems like I can’t go much further. I hit this ‘brick wall’ and I don’t have any energy left. I’ve never tried jogging much past about 2 miles. Last week, I went jogging for just over 8 miles though. It was along a hiking trail and I had to pause after a mile or so when my foot cramped up. But after 5 or 10 minutes, I set out jogging again and I was fine. Periodically, I’d have to walk because the trail was too rocky or I felt out of breath. But in the end, I jogged 8.5 miles in less than 2 hours, which for me is a first. I thought it was very strange that I was somehow able to jog so far without hitting that brick wall. I attributed it primarily to having to walk at points along the trail and I caught my breath so today I tried an experiment. I went for another hike/jog today. This time I hiked out 4 miles (to get warmed up) then jogged back. I got on a different section of the trail that had a nice, flat road I could jog back along so I didn't need to stop to walk along rocky sections. After the first mile or two of jogging, it gradually got easier. Not easy but I seemed to get past this ‘brick wall’ that’s prevented me in the past from exceeding 2 miles. The first 2 miles takes about 20 minutes. That seems to coincide with another observation I’ve had while working outdoors in the winter. After about 20 minutes I feel about as cold as I’ll ever feel. Within another 15 minutes or so, I’ve found my body tends to warm back up without added exertion. The question is, could there be some biological mechanism or biological process which provides ‘short term’ energy, say up to 20 minutes of hard excertion and after that it essentially runs out? Once that first biological process uses up all the energy available in some form of chemical energy, could it be that there’s a second biological process which the body switches over to in order to provide an energy source for muscles? Or could there be some other explanation for the observation?