I average a little over 100 miles/week these days. I've done a couple of centuries this year, but I'm just not strong enough or comfortable enough in the saddle (even on a comfy bike) to want to spend enough hours to make a double.
Someday I may train for a double, but for now I'm more interested in training for climbing centuries next year. I'd rather have a jersey that says 'Breathless Agony' or 'Death Ride' than 'Hemet Double'
Air, any pace you can manange to get you to 100 miles will obviously get you there. It's just about your comfort in the saddle. I'm good for 7 hours or so in the saddle. After that, I'm pretty much done. If you can manage longer, go for it! I did my first century with Team in Training and we had someone come in at 13 hours!