Let me guess. You'd done similar climbs in past years without a problem. Now, in March, in the first decent weather in New York so far this year, you've gone from nearly no riding to attempting climbs like the one you've described, and you expect such climbs to feel exactly as they did in the middle of the season last year.

As a self-anointed "Experienced Cyclist," you should know better. In my experience, as a rider who started racing a track bike in 1964 and has done innumerable miles since, I'm proof that you can quickly work your way back to riding as you did last season, and without the need for doing anything other than riding your bike. Yes, I've sometimes experienced transitory lower-back discomfort on climbs in the early part of the riding season. But I've never lifted weights, I've never stretched, I've never worked on my "core." No need. Just ride the bike, and you'll be fine soon enough.