Around 16 miles if you stick religiously to the edges. However, it's very easy to cut corners and it depends on exactly what you define as the "edge" of Kinder. When I lived in the Peak, used to do it once a year. Lots of interesting bits on the Northern edges e.g. Boxing Gloves.
Ironically the Barre was going to be a warm up route for my buddies and I to film and traverse the mighty Matterhorn via the Lione and down the Hornli but was so out of condition we went to the Aigulle Dibona instead! Hopefully going back for another rematch this summer...
Better luck luck this year! The climb was meant to be out of condition when we did it, instead it was us that was out of condition. We did the whole Italian side wearing crampons, but it was good neve. The only really hard part was where I struggled over an overhang, only to discover when I landed on the ledge above that there was a big iron ladder around the corner.
Great video. How did you manage to get the close up of the Marmot? You made the right decision not to continue. On my last big Alpine route, the traverse of the Matterhorn, we faced a similar dilemma. I hadn't climbed for almost a year and went straight from the UK to Zermatt where we walked under the East? face over to the Italian side. As we were pressed for time and there was only a short good weather window, we set off up the Italian ridge immediately.
Once we got above 4000m progress slowed to a crawl and I felt really unwell. We pressed on past the summit and ran out of daylight somewhere above the Solvay hut and ended up spending the night sitting on a sloping ledge. Since were were travelling "fast (or in our case slow) and light", we had no extra clothing or bivvy gear. Luckily there was no wind and the night was spent with 20 minute bursts of shivering, which warmed us up for a few minutes until the next shiver fest.
We got off OK the next day and once I got back below 4000m I felt much better. However, we should have turned back at around 4000m on the Italian side and rested for a day before trying again.
Have a look at v-g's site. He parks up in Dolgarrog and walks from there. However, it's a long hard slog. There is very limited parking at the top of the steep bit, but you would probably need to check with the farmer that it's OK to leave you car there.
I think that I would be more worried about a hard freeze turning the steep part into a skating rink. There is a farm beyond the big parking area near the reservoir, so the road will probably get cleared quickly if it does get blocked. Definitely worth keeping and eye on MWIS to see what the snow line will be.