My concern would be injury as you add on additional miles. I am an army veteran and have run in my boots many miles (although not a marathon). I, personally, wouldn't train for a race in them.
There are other ways to build strength. Speed work, hills, weight training. These won't stress your body quite as much.
Find the right shoes for your body type and running style. I pronate- badly. Flat footed from the military. :-) I train, and race, in Brooks Ariels (Beasts are the men's version). A good, specialized, running store should be able to help with shoe selection. Online: road runner's sports has instructions on how to find a good pair of shoes.
Good luck with your first! :)
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