At St Athan (School of PT) in the early 60's I was told the measure of fitness was recovery rate - if you run 100 yds your heart rate should reach about 124 BPM (once a day, this is beneficial according to the experts) - however, the time taken to stabilise back to 60-72 BPM is more important, and is a true measure of your fitness - the lower the better!
Stamina is very different - it's the ability and will to keep going when you feel shagged, and is developed by lifestyle - diet, exercise, determination and aspirations, plus the need to win and succeed
Ranulph Fiennes had a couple of failures in the Antarctic - but later, went on to run 7 marathons in 7 countries in 7 days after a major heart operation
He was lucky though - he didn't have a PTI with a bleep meter shouting "Go for it, give it 10"
Merry Christmas
B15