Generally, 50 degree air is not cold enough to effect most people unless they have a sensitive respiratory system. Your body can easily deal with all temperatures above freezing as long as you are warmed up properly and you have adapted yourself to it gradually. The colder the air is the more dense it is. So while it is harder to ride through it also packs more oxygen per cubic volume which should help your aerobic system.

Generally, you just need to go out in the cold on a regular basis. But ease into it at first by not doing long intense workouts until you get a little used to it which may take a couple of weeks. If you have been going out several days a week as it cools your system will easily adjust to it with no problems.