Its been discussed here several ways, but I am in the very strong camp of climbing is considerably different than flat. I live on a road where if I go one way, I have a 2500 foot climb in the next 7 miles, or I go the other way and I have a 4200 foot climb in the next 9 miles. Or I can go N/S and stay under 1500 feet of climbing in 40 miles. Long sustained climbs require substantial power (constant) over all that time. I am slow, and about 220 lbs. I cant really go less than 180 watts on these hills or I fall over. I have to average around 190-200 on the climbs (50/34 - 12/28) What I mean is if the climb is 2.5 hours to the top, I am cranking 200 watts for 2.5 hours....that to me is WAY different than any fast flat ride I have ever done. I always find a way to take a break on the flats, or catch a draft, or something.
I am a rank beginner, so take what I say with a grain of salt, but if you have only ridden flats, and you want to go climb 9K feet...be ready for a world of hurt.