Cold days make me "tight" on the bike as I gather myself in. Also probably a little constant muscle tension and stiffness that nibble at energy, plus keeping warm, plus generally a stronger wind in your kisser (and down your neck) ...all contribute to wear one down faster. January days are good days for long, slow rides enjoying the scenery. Not the time or weather, not the time of year or time of training cycle to push hard. Save it for sweet April and beyond. But stay out there--- makes that postride hot shower all the better.