For a short while The outward journey sorts out the riders who aren't as fit. If the pace is reasonably steady then a strong C grader should be able to last until the second climb of Blood, at which point their legs and their brain will have a formidable battle. The legs usually win by creating such large doses of lactic acid that the ego has to submit

[edit] PS. not many A graders in our bracket, but some very strong B graders who verge on being A grade