Be careful with that feeling and don't underfuel, or at least keep it at "sane" levels. I feel pretty amazing and full after 62km/2700mD+ XCMs as well, as an extreme example... which is at least partially due to the immune system (and resp. inflammation/etc) being suppressed. Long, light/moderate efforts without adequate food intake and rest can lead to the same thing.
Stress hormones. Read up on cortisol's effects. XCM - cross country marathon (MTB race). 2700mD+ - 2.7km of vertical gain. A reduction of inflammation is a general effect of anti-inflammatory drugs, which tend to make you feel better. There's a LOT more than that (i.e. say, all the things that fall under the umbrella of "runner's high"), but the TL:DR is that significant physical activity / energy expenditure, combined with a lack of proper rest & nutrition leads to long-term undesirable effects that can definitely be masked and/or disregarded.
