It's enormously important. I was off the bike for three months last year from a combination of work stuff, then a gall bladder operation. My conditioning had to go downhill in that period, but I also lost about 30 pound. When I got back on the bike, I was short of breath and sort of weak, as you'd expect--but my climbing was better than it's been in years. I was a full chainring higher on one 1.5-mile grunt near my house--from the granny and big cog to the middle ring and big cog.