You can try different things and see how your body reacts but the variety is good also. To change your workouts every 4-6 weeks is a good way to keep shocking your muscles to achieve best results.
As a (very) general guide
1. from 1rep upto 6-8 reps would be to increase explosive power so great for sprinters.
2. 6/8 to 12 reps will increase size of muscle most and therefore strength...
3. 15reps plus will increase muscular endurance, and improve the ability of the muscles to perform exercise where strength is required over longer periods of time.
Obviously all will increase "strength" to some degree, but by doing more sets you'll teach the muscles to use oxygen more efficiently when doing resistance exercises, so you'll be actuallydoing a cardio workout whilst weight training.
Try this for one session and see how you get on....the general rules are 1. don't rest for anymore than 60 second in between sets 2. If you cannot complete the set, continue reps until you literally cannot lift anymore. 3. The weight is unimportant, especially at first. You may find yourself not even using any weight to begin with. Each set will seem very easy until atleast 30% into it beacuse of the low weight compared to what you're used to.
Warm up full body. Focusing back and legs.
1. Leg extension (with machine) 3 sets x 50 reps (great warm up for quads for a full legs workout)
2. Squats - 3 sets x 75 reps (with smith machine, using only bar)
3. Lunges (with small dumbells or without) 3 sets x 100 reps (50 each leg per set) / steps ups same sets/reps
4. Deadlifts (low weight) 3 x 50 reps
5. leg curls (machine) (lie down or sit up) 3 sets x 100 reps
Warm down/stretch.
Protein shake an absolute must. (approx 40g-50g). + carbs
It will seem really strange at first and wont be easy, forget what weights you usually use, better to start very low with the weight and work up. Teach your body to drive through the lactic acid, because it will come almost every set. What you're looking to achieve is getting your muscles to use oxygen whilst under pressure at the maximum efficeincy. It works.
Core is really important, I don;t think anywhere enough runners put enough emphasis on this, if you're happy with the above, I'll send some information on ab/core workouts and upper body.
I hope this helps 