Well said......there is a huge difference between footballers in their physiological profiles. The fact that Aaron Davey and Sam Blease struggle as soon as the endurance requirements build will have almost everything to do with their specific characteristics. Basically at either end of the spectrum you have pure sprinters, whose muscles have the highest percentage of 'fast twitch' fibres, at the exclusion of 'slow twitch' fibres (think Gary Ablett snr.), and pure endurance athletes, where the fibre typing is the reverse (marathon runners, the Schleck brothers of Tour de France fame). There is any variation in between these extremes, and numerous other factors come into play, such as height and limb lever lengths, which will affect specific abilities. As a rough generality, heavily muscled physiques (Moloney) will tend to have fast twitch predominance, as those fibres have a greater capacity for hypertrophy (growth), and skinny guys (Morton) the opposite. There is a misconception that skinny cultural types such as indigenous australians, are naturally endurance favoured, but I would suggest the vast majority of AFL indigenous players are fast twitch, 'burst' athletes, and given that they are represented in the AFL at 8 times their representation in the Australian population, they enjoy quite an extreme physiology.
Every body has a percentage of fibres that can be influenced or trained either way, and a fast twitch athlete can improve their endurance, and slow twitch can improve their strength/power capabilities, but to a varying degree. Endurance abilitites naturally improve through the 20's and 30's, then slowly drop through middle age, which is why you see experienced players able to go longer through a game, and another reason why maturity is essential in your playing group (look at Geelong this year). Of course mental experience is also essential, but it is still largely physiological. This is basically related to an individual's 'VO2 max', that is, their body's ability to take in and use oxygen efficiently, and their 'lactate threshold', which is the point at which the body starts to rapidly develop lactic acid which will curtail muscle performance. We all have a genetically determined base level, and the rest is training and time. For example, Cadel Evans has the highest VO2 max ever tested at the AIS, and he was still a mountain biker at the time. He was born extreme.
The fitness and coaching staff will be COMPLETELY aware of the characteristics of each player on the list, and their goal will be to maximise and make use of their extreme abilities (Davey and Blease for their burst speed), and improve their other end (ability to go all game) as best they can without risking compromise to their more unique talent. The fact that Davey and Blease couldn't stay with Leigh Williams merely tells me that Williams is probably more an 'all rounder' and will lack the special talent of either end. Thankfully, football requires a whole bunch of other abilities.
As an illustrative anecdote, and one which I loved at the time, I was cycling a leg of the Tour de France in 2006 (organised for hacks like me every year, to see how the big boys suffer), and on the first 15 km climb of a 190km stretch, when I casually wheeled past Sir Chris Hoy, (he was just 'Chris' at the time) the Gold medal winner in the Cycling Sprint at Beijing 2 years later!!!. He had the biggest thighs I'd ever seen, but they were seriously struggling. He was interviewed after the event, quoting that it was the hardest day he'd ever had physically. I finished an hour faster than him, and am myself burdened by athletic mediocrity.......... horses for courses.
On the topic of physical extremes, a colleague was consulting big Max Gawn a couple of days ago, the morning after his ACL repair and getting him upright for the first time, and wasn't quite prepared for his height (apparently he is smaller lying down!), to the point where she let out an exclamatory ''F*** you're big!" . Anyway, he's on the road back, good luck Maxy.