Hi Ladies!

I've been reading the forum off and on (and occasionally posting) for a few years, but I have quick fitness question for ya'll. Last few years I've been mostly riding road, 20-30 miles of flat once or twice a week and commuting to work 5 miles each way a few days a week. I feel like I'm in decent shape for what I ride. However, I just got back into mountain biking (thanks to a brand new santa cruz juliana, she's GORGEOUS! I love her), and on an hour ride with lots of hills(Wissahickon park in Philly), my legs are fine, my lungs are KILLING me. I just don't ride hills that often!!!! I'm trying to jump start myself aerobically, maybe I should start running on off riding days???? Any suggestions for getting my lungs up the challenge? (other than riding more hills! )