It's a real mixture. Some is tarmac, some is easy gravel, some is a little bit rough.
I did it on 28mm tyres - the exact same ones I had for the TCR in 2019, in fact. Most of it was rideable with care on them. For a few bits they were clearly a stupid option, and I walked a couple of small descents because wrong tyres plus hardly anyone about plus no phone signal (plus lack of skill on my part) meant that if I did come off and hurt myself it could be a problem.
It's a real mixture. Some is tarmac, some is easy gravel, some is a little bit rough.
I did it on 28mm tyres - the exact same ones I had for the TCR in 2019, in fact. Most of it was rideable with care on them. For a few bits they were clearly a stupid option, and I walked a couple of small descents because wrong tyres plus hardly anyone about plus no phone signal (plus lack of skill on my part) meant that if I did come off and hurt myself it could be a problem.
There's a GNT thread on here with loads of info, and link to a blog with some great pics.
https://www.lfgss.com/conversations/351273/?offset=25#comment15824564