How do you prepare for 5000m of climbing?

I've ridden 188 miles in the last six days, one of my best weekly mileage totals of the year. Targeting the hills I have amassed 3600m of climbing - but this is still only 72% of what I'll need to cover in a single day on the 20th August. In fact checking my Strava history, my best ever weekly altitude gain total is 4344m. My monthly totals fairly regularly fall below the 5000m total of Grand Raid! If it wasn't a challenge what would be the point?

Grand Raid route profile.