Europa hit the nail on the head.
From the 'been there done that' side of New York State and 'over 50', I've far too often gone out with the A crowd and paid for it -either with a nasty 18mph wind on the way back or a climb that went well over a half mile.
The body uses up the oxygen and the carbos and..here comes da bonk. (push it too far and the bonk becomes 'da wall')
There are guys (and gals) I ride with who are easily my (mid 60s)age and have the leg to go and go and go regardless of the terrain.
I'm not one of em and as Clint Eastwood said: "Man Needs To Know His Limitations"