Excellent! I looked at the map last year, thought "meh, I did the Fred Whitton Challenge at ~20kph O.A, how hard can it be?"
Harder, that's what.
But, you have to remember, I am slower at controls than you and often finish rides behind you. So you'll be fine.
There was talk of it being the hardest 300 in the calendar. Is that true, or just wishful thinking?
Anyway, my training advice would be to do the Severn Across, Fred Whitton Challenge, Bryan Chapman on consecutive weekends, but it's too late for that. The MSG has much shorter climbs generally, and they don't get as steep by any means. You also have to remember that the first 50km will be fairly flat, as will the final 50km. Only the middle 200km is more hilly than the Fred Whitton, AIUI (4800m in 200km vs 3500 in 175km). The calendar page from last year's event says AAA 4.25.
If the route is reversed, then instead of going up the easy side of Holme Moss at 140km just as the 140km low energy point hits, on an empty tank, you'll be going up the hard side with a stomach full of food from the cafe at Holmfirth.
Is any of this helping yet?
As for Mike. MSG 300 as your first 300. Well, you've got balls, I'll give you that.