You will be fine, it's about 30 minute from Gate C to the furthest field in General. There's a lock up near the gate once you are through the ticket checks. Its a good place to leave warm clothing, wellies, booze to save a trip back to your live in vehicle.
Sex Pistols + Frank Carter playing two gigs during Glasto... one is Jera on Air in the Netherlands on Friday 27th June, and the other, at Tons of Rock in Norway, is annoyingly listed as "Thursday 27th June"... I assume the Thursday bit is correct, as they are already playing on the 27th!
So doesn't rule them out for Glastonbury... does this make them more or less likely though?! I'm not sure! (and they are already doing Download)