To be riding for 29 hours is nuts. In Nov I did a 24 hour Iron Butt Challenge and was dead tired after that, I can't imagine going off road for a longer amount of time.
I don't pretend to speak for Phil but maybe he can confirm my experience. I've found it easier to ride endurance events if they're off road despite the fact that this type of riding is much more physically demanding. First, I tend to take off road riding much more seriously and eat and hydrate much better than I do for a day on the tar. Second, when riding off road, I end up never allowing a single part of my body to get over-fatigued like I do on the street. My entire body may get much more tired wile riding off road but my butt never suffers like it does on the street. Finally, riding off road is much more exhilirating for me because of the constantly changing terrain. You just dont find youself constantly struggling to stay upright when riding on the street like you do when off road, especially if you're racing. Maybe I'm just an adrenaline junky (I've nearly passed out after a race or two when I finally came off of the high and I was so exhausted that I had to be driven home - couldn't drive myself).
I've done Vegas to Reno and was in pretty good shape after 12.5 hours. I did a long dual sport ride in Death Valley and Saline Valley for about 15 hours and the only reason I was wiped out was because it was August and the heat...OH, the heat!
Riding for 4 or 5 hours on the slab and I'm done!
08 KTM 320 SXF, '07 Beta Rev 3,
Frankenberg 501, '96 Husaberg FC600, '68 Yamaha DT1, '74 Bultaco Alpina 250, '84 Husqvarna WR400 - My Dad says I've never seen a motorcycle I didn't want to own...