Paul!!!!! With the greatest respect the BGR cannot be in anyway be compared to doing the TdF.The Bob Graham round? A triathlon? The Welsh 1,000 metres? Okay, they're not sustained over anything like the time frame of the Tour but I would estimate all of those events would be harder than even the toughest TdF stage taken in isolation. And cycling is a lot kinder to the body than anything involving feet/ground interface.
While I have immense admiration for anyone who has completed it, the effort required to basically crawl up 42 peaks in the Lake District and jog down the other side is a drop in the ocean compared to 3 weeks of effort the cyclists do.
I see I may have misread your post and yuou're comparing the BGR to one stage. Stand at ease.