Bowen and Burke were mere lightweights though. (Bowen stopped Jack Everhart in 85 rounds the very next month though! 195 rounds in two months? That's a career's worth for today's over hyped and pampered pussies. Hardly a surprise that Bowen was killed the following year.) What heavyweights Corbett and Jackson did over 61 timed rounds is perfectly respectable. Sullivan-Kilrain went two hours, 16 minutes and 23 seconds, and it was 104 degrees in the shade at ringside. Sullivan was already past prime, and actually overweight for this showdown, despite the intense training. At 190 pounds while in his mid 20s, he must have been a truly frightening adversary for any opponent given the time to take stock of him.
The vast amount of in-ring experience also accounts for a lot of it. When you're fighting full bouts as often as modern day fighters are sparring you're bound to have an advantage in that regard.
Since taking up ultra running I have found a new respect for champions like Fitzsimmons and Jeffries. The running component of their training regime was impresive in itself.
ULTRA RUNNING?? What's that?? I took up running back in March - just 10k's so far - but I'm coming along Ultra Running sounds a bit mad!!?