For a successful weight-training experience, keep the following guidelines in mind:
Warm up before lifting and stretch thoroughly afterward.
Run prior to lifting, and avoid weight training leg work on days before races, speed workouts, or long runs.
Lift every other day or a minimum of 3 days per week.
Emphasize lighter weights and more repetitions rather than heavy weights for a few reps.
Don't hold your breath while lifting weights; breathe in on the relaxation phase and out while performing the hard part of the exercise.
Move your body through the entire range of motion of the exercise, making sure you don't lock your joints while performing the exercise.
Follow the sequence of legs first, upper body second, and midsection last, remembering to work your abdominal muscles.
In each sequence, exercise the larger muscle groups first, followed by the smaller groups.
Remember, you probably won't lose weight as you incorporate a weight-conditioning program in your present training. Instead, you will gain muscle and lower body fat (assuming you eat sensibly). Thus the scale can be very misleading. As you lose fat and gain muscle, your clothes will fit better, and you'll look and feel great!
Run First, Lift Later
Runners should ideally run first and do strength training second, preferably not back-to-back. The best thing to do is schedule several hours between a run and your strength workout. You may run in the morning and then do your strength routine at lunchtime or in the evening.
If you are forced to perform the two routines together, do your run first and then your strength training. If you're doing a long run or a speed workout, hold off on the strength training afterward. You'll probably be too tired to perform it properly.
Some have recommended that you perform your hard running and strength training on the same day (but separate the two), followed by an easy run the next day so you have time to recover. Experiment to see what feels right to you. You might find it easier to do your strength training on a light running day or even on a rest day. For more advanced runners, if you do strength training on a rest day, go very easy on the legs or skip the leg workouts entirely if you will be racing or doing a speed work session the next day.