I did Leadville last year on a hardtail 29er.
I arrived on Thursday evening, picked up my packet on Friday, did a short ride, and went to bed early. Race on Saturday went pretty well... got to the top of Columbine in 4:30ish, but at the 7 hour mark my stomach shut down and I lost over an hour in the last 10 miles. Finished in 9:53, an hour slower than my goal.
Anyway... I never really felt the altitude cardiovascularly or aerobically, but it could have contributed to my GI issues. So, make sure you're taking on energy/calories AND plain water during the race. Too much of just one can backfire. Badly.
My teammates for the race were current NAVY SEALs who have spent a lot of time acclimating to altitude in Afghanistan; their advice was to arrive as soon before the race as possible (i.e., the day before) or to spend 3-4 weeks acclimating to the altitude. Anything in between would likely hurt your performance.
You could also get a Rx for Diamox, as prophylaxis against altitude sickness AND altitude induced pulmonary edema - getting an IV and O2 after the race is a total bummer.