If you want to wean into it it's really quite simple, I used to race xc biking and dh all that good stuff. Running was one of the main parts of my training regime. Do intervals. If your trying to get back into start on a track somewhere, run 15 seconds hard 45 seconds jog, then 20 seconds hard 45 seconds jog, 25 seconds hard 45 seconds off, until you reach 45 seoncds hard. That's about 8 minutes and 10 seconds of consistant running. This not only will help improve your bodies ability to remove lactic acid but also improve overall muscular endurance(which is obvoiusly caused by the build up lactic acid). If you run like this it will beat the shit out of you. But the rewards are much greater than just going for a long jog. Don't pay any attention to trying to maintain your heart rate at the optimal rate. If you do this I guarantee you that by the end of a month ( based on running Mon. Wed. Fri.)you will be able to increase your intervals all the way up to 2:00 full sprint. As someone said before, dont forget to lift as well. Another option you might want to try is the stair master. People call it girly, but u burn more calories on that then you do running. It's also less impact on the joints in your knee and still presents good cardio, there is also usually an interval option on it. That is what I do now for 45 minutes everyday after each workout at the gym.
