I think they would be the same as any Audax event if they were done along surfaced roads. But I think they are probably more like doing Audax distance rides on a mountain bike and off road. If you've ever been mountain biking you'll know how much harder it is to do the miles off road and usually with much more climbing.
I guess you'll have to find that out for yourself.
All I know is that there is no freewheel for your legs when you are on foot and whilst I am sure you are right about covering Audax distances off road being harder,
Riding a mountain bike, even on the road is tough going. I cycled to Dolgellau in 2006 on my mountain bike on a Bank Holiday weekend, to meet a friend to go mountain biking at Coed-y-Brenin. It was about 170 miles which would have been an easy 10-12 hour ride on the road bike. It was a long slog on the mountain bike and I was still very tired from it the day after when I was riding with my friend, but because my friend had his girlfriend with him and she was riding with us I didn't need to be fresh, but I still had to spend a while in the cafe reviving myself while they rode one of the shorter trails.
Then I cycled home on the Monday. I realised that it would have been faster and much easier for me to ride up on the road bike and just walk around the mountain bike trails, then ride home again.
Add to that that off roading is harder still than road riding because of the rough surfaces and extra hilliness. I know that I'd not find it easy to cycle 600km in a weekend off road. Even just a 100 mile ride without much climbing is quite a tough day on the mountain bike when I'm fit.
But yes, I'll have to suck it and see. I'm not sure I'll be able to manage a 100 miler this year though.
My mate Tom took almost the whole 48 hours to complete the same event but went on to do over 10 consecutive 100s, some harder than the Dartmoor 100 by quite a margin (Snowdonia 100 - 30000ft of ascent). In the end, he didn't bother to train for them because running, walking and cycling everywhere became a pivotal part of his life so 'training' meant nothing. He stopped the 100s in the end after a series of injuries led to him questioning why he was doing them - and not liking the answers.
That's the thing. It's what I tried to do in 2007. Just start easy early in the year and get myself doing the distance and naturally get fitter. The toughest rides, or at lest, the ones that hurt the most were the ones from January to March. I was increasing the distance of the rides then, I wasn't up to speed and the weather was usually very tough. Lots of strong headwinds and often into driving rain and dark for most of the rides. I remember reading about people struggling to ride into those winds for 100k where I was riding into them for 400. But once the weather improved (a bit, it was a pretty crap year for weather) and I recovered, it just became business as usual. This meant that I could recover more and get even stronger so that I was doing mega miles and still sometimes in bad conditions, but much more easily. These rides I was doing mid-summer quite comfartably would have been impossible for me in the early season.
It's sort of why I'm giving this a go too. You say you're not anything like as fit as you used to be, but you were going very well on Porkers and Brimstone last year. You can get up the hills fast for a big chap!
I'm wondering whether it's that you are a lot fitter than you think you are because you've just got into the swing of things or if this long distance walking really is all that tough. (I reckon that walking or running is much tougher than cycling because of the impact issues, but fitness wise about the same. If anyone tells me that travelling by foot needs more cardio fitness than cycling then I'll tell them to cycle faster.) After my half marathon (even if it was a very fast course) I'm starting to think it is the former.
But I want to find out for myself and it's something that appeals to me, so I'll give it a proper go. I wont suffer for doing it. If I stop enjoying it or start getting too damaged from it, then I'll stop. It's just a side show to my cycling which will always come first.
I am very happy with the Audax stuff now as it offers a reasonable challenge, is more sociable, keeps me fit and I get to see some great countryside without killing or injuring myself in the process.
That's good, but be carefull, you
can do too much of that Audax game.