Seek out climbing on a regular basis and push yourself harder in the hills. If you mainly ride on flat-ish ground you really lose your climbing legs pretty quickly. I don't think knobby tires are that much of a speed drag below ~15mph and if you're sustaining 15mph+ on a hill it's too easy to consider that climbing (more like an incline.) And yeah, don't expect to erase a 32 year age gap with a tad more training.