This year I started riding up Mt Diablo in the CA Bay Area. It's about a 10 mile climb of 3,700 ft gain (ending in a 17% grade) but you start nearly at sea level. I also started riding Iron Mtn Rd (Mormon-Emigrant Trail) in the Sierras which is about 21 miles and 4,000 ft gain so it is a shallower, more gentle climb than Diablo. However, it goes up to 7,000 ft altitude. I think that plus the double mileage makes it harder. Maybe it is psychological but somehow Iron Mtn just seems tougher.

My avg speed on these two rides (including the downhills) doesn't even come close to what I maintain on the flats during the week but my speed on the flats has increased because I now ride some serious hills. It's also changed my definition of a "tough" ride.