Sorry for the above.
First of all, well done for getting a significantly good time of 38 minutes at 10km, which is actually 6.25 miles as near as dammit. The extra quarter mile does make a difference - 90 seconds if you think about it.
The question that you have to ask yourself here, is how much to you really want it? You say that you are a boxer who runs 3 times a week. So are you just trying to add a dose of additional fitness to help your boxing? If so, you can forget about the numbers, they will be what they will be.
If you are a runner who occasionally boxes, then it's a different matter.
Also, if you are built like Frank Bruno, congratulations on getting as fast as you have. If you are built like Amir Khan then there's some hope of changing.
My personal opinion is that you would need to step up from 3 days a week running to at least 6, with some days being double sets. Obviously that's a big change and will take a few months to get through.
The simple truth is that to run faster you have to train faster. To run for longer, you have to train for longer. For mid distance speed I was recommended schedules in a book by Jack Daniels.