I'm a scaredy cat when it comes to climbing so I understand your dilemma. However, it's my lungs and not my knees that give out on me. That said, I you gotta start somewhere with this year's training. Despite my own doubts, I was planning to do a hilly 60-mile ride in Southern Indiana this coming weekend until the weather turned to-you-know-what.
Are your knees currently bothering you from riding in the flats? If so, I'd say you might want take it easy for a bit longer before taking on any big climbs. If your knees are doing okay, I'd encourage you to do the ride but go easy. Don't try to power up the hills. Assuming you stayed in decent shape over the winter, you'll do just fine. I find that when my knees start feeling a little tender, a few IT band stretches on a daily basis, ice and Motrin usually do the trick. So you might want to follow that regimen before and after the ride.
Good luck!
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher