When I was at my best level where I could run a half marathon after a week of shifts without worry I did it by just building my running slowly, knowing when to take days off and when to rest and recuperate. It just takes time.
There are specific exercise plans all over the place and all sorts of strengthening exercises for running, including importantly core strength.
I'm sorry I can't be more specific, but this topic is the basis of a hell of a lot of articles, books etc.
Yeah 50 miles. If you fancy a bit of fun bed time reading, Eat and Run by Scott Jurek Where Scott tells his story of becoming and being an Ultra runner.
Some well conditioned people can run 50 miles in a single run and do it again the next day no problem.
On the other hand if they can't swim very well they might drown!
The point is this is a very individual thing. Running will in general put a lot of strain on your body and people have a tendency to do too much too soon. Cycling and swimming will take a lot of pressure of joints etc but will take a lot more time to reach the same amount of energy expended.
Best advice is to make sure you fit in your rest days and listen to your body.
Just keep pushing it little by little would be my advice. Set yourself your next target time maybe just 5 seconds off and see how close you can get to it across your whole run. Then when you achieve it push on another 5 seconds.
It's the small increases over a long time that helped me when I first started. If you don't put that little extra in whether that be distance, speed, intensity you will stop developing.
I am also not a big fan of Piers' advice to increase stride length. The longer your stride the more likely you are to heel strike the ground out in front of you which adds a braking force to your running which goes right through all your joints. If you want to add speed it is far more beneficial to run with light foot steps and just pick up your feet more frequently. It feels weird at first, but will aid you greatly in terms of speed.