Exercising makes you strong, but every exercise requires a certain amount of strength in order to actually do it. This leads to a bizarre catch-22, because you may not be able to do the kind of exercise you want because you are not strong enough, but were you to do the workout, it would make you strong enough.
This problem shows that a lot of physical training is getting your body to the point where you can do the workouts you want. It is sort of an intermediate step in achieving your ultimate fitness goal, but one that can be critical. Here is an example: to push your lactic acid threshold, you need to run long intervals at high intensity with short rests in between. This workout could be repeat miles slightly faster than race pace, with four or five minutes of recovery where you run considerably slower than race pace. This is a very demanding workout, but one that pays huge dividends in fitness. But actually running that hard that long requires speed, strength, flexibility, and endurance. Ironically, these are all things that you would gain from doing the workout.
The solution is to build from one workout to the next, until you are ready for the big stuff that you really want to do. Before you do intense speed-work, you need to first have endurance. This means doing long, steady runs until your cardiovascular system can handle that level of intensity. Strides would also be a good idea, because you wont be able to run fast for long until you can at least run fast short. Start on a track where the ground is smooth and work on your form until you feel that you can run fast well. You also need strength, so make sure to do some kind of weight lifting and keep those new muscles limber by stretching regularly.
This path may sound lengthy, but you have to move carefully to avoid getting injured. I recently heard an interview with a professional runner who was doing 110 miles a week, and he said that it took him years to get to that much mileage. He would increase his mileage slowly, allowing his body to adjust and grow into the higher volume. I could tell that he was looking at his running career as a whole, and making decisions that would pay off months and years down the road. This "big picture" view is important, because it shows that the preliminary steps are just as important as the final steps. And this can motivate you when you want to be running a marathon, but right now you should be doing your stretches.