OP - just curious, let's say you are getting dropped by a faster group, what part of you is breaking down when that happens? Is it your heart (chest pounding, HR at redline, world compressing into a tunnel), your lungs (gasping for breath, tongue hanging off your chin, blowing like a narwhal), your legs (muscles burning, legs locking up, limbs won't respond to the go command - and which muscles?), or your will (which is unlikely, for you)? That might also be a useful clue.