Paul, 30C makes a bit more sense, I was only guessing! Hydration was important and I got through 6 or 7 bottles and a couple of pints of milk., which may not sound a lot but I ususally do a 200 on a couple of bottles and a pint of milk. It's still a great ride though. I think you'll get round ok. You're quite a bit younger than me and a strong rider and I think you have the right attitude: just ride it and if you "qualify", fine, if not, it's still a fantastic route. That was how I approached the Old 240, and it worked for me.
Did you do your recce of Wrynose and Hardknott? By the way, I expect you know this, but if the 600 takes you up Kirkstone Pass from Ambleside there is some really good Coniston beer in the Inn, though you are morely likely to be in need of a blood transfusion!
All the best
Peter