it always takes me about 10 miles to get warmed up no matter what the temperature. Since I have floating cartiledge and am missing the ACL in one leg I do however always make it a point to keep my knees warm with knee warmers or tights. The hard part about riding out into the cold is that although you may need layers to start, you warm up from riding sooner rather than later. Knee warmers can be easily removed and stowed in a jersey pocket. But then my tights all have zippers in them so I can also unzip and roll them up to look like knickers and ride that way.

Having had injured muscles, tendons and ligaments, I would advise taking it very slow and easy and be careful not to let yourself get cold while riding.