Metrically challenged

I just signed up for my first metric century. It's a bit of stretch, but now it's feeling like a real stretch. I know it's impossible to ask if I can do this or not (it's in Sept.), but what is the best strategy for training? It's got three extended climbs that are 8- 10% 2 miles in length, another that's over 15% for a mile and one that's a mile and ranges from 10-26%. The rest is very rolling. It's 6k of climbing overall.

My regular rided usually consists of 20-30mi with 1500-2000 feet of climbing. The most riding I've ever done on a road bike is 40mi and 3,000 feet of climbing this June.

Any advice or training resources?