Fitness

Not a fitness expert by any means, just an enjoyer. But from what you are telling here you need to improve your explosiveness/power. That will make you jumpstart faster to your highest speed. Maintaining it is another issue, but could be done by many means.

To improve your power, I would go with what another user suggested. Do some kettlebell exercises. You can also do some thrusters with dumbbells, or power cleans.

To maintain your speed, I don't know. I'm not an expert, least in running. But I noticed that the more I lift (in terms of raw weight) the easier is for me to run.

Of course, this is disregarding technique, which is key. You can have the best legs in the world, that without proper technique you'll get so far. Conversely, if you have the proper technique you don't need to improve your fitness THAT much.