I hope your preparations go well. I am sure you will find the Sherpa an excellent choice, more and more it seems to me the features that matter are good basic design and geometry, and simple but decent quality components.
I have just got back to UK from a sponsored London - Paris ride on the Ripio I bought last year. The 'tight' frame and low crossbar (sorry, top tube) of mountain bikes has always suited me, and the Ripio is probably pretty similar to the Sherpa, maybe a bit shorter in the chainstays.
There were 15 on the ride. All the others had modern dropped road bikes, some with tyres like rubber bands, others using hybrids. Mine was the only one with 26" wheels. It was the heaviest in the group, but in reality not all that much more than several of the hybrids - mine has 2" Supremes at present and solid Mt Tura forks, so the kit is much the same as your Sherpa.
How did it go?? The bike went faultlessly, rolled along the level and downhill just as well as the lighter bikes. And people were surprised at this - " My bike shop told me 700 wheels were much faster!!" Not true.
Yes, it was slower off the mark at junctions and so on on the urban sections, and slower up the hills, and I found my pace just a bit slower than others, which was frustrating as I either had to push harder than I like which knackered my joints a bit, ride alone, or feel I was holding others up. But that is down to me as much as the bike. At one time, the faster riders formed a pack around me and swept me along at 25 mph plus - what an experience!!! Riding alone, as I think you will be doing, and riding in a group, is a very different experience.
And my bike excelled, of course, on the more 'trail' parts of the ride, with gravel surfaces, where the others were in difficulties - though the Supremes, brilliant on tarmac, were not good on the slippery muddy bits at the sides of the tracks. Duremes much better for that. No punctures, brakes and gears all perfect . . . . . At all times I felt secure and in control.
So the Sherpa will look after you I am sure.
Lewis