Choosing a 10K as a stepping stone to a marathon is a great way to assess your base pace and fitness for training.
Though it may seem counter-intuitive, my suggestion to help you both increase your running pace and distance is to run more miles, and run them more slowly. I really like this article from On Running about why slower running can help promote faster running on race day. It's incredibly detailed and discusses two main factors that can contribute to faster runs--how your body uses up its energy stores, and how your muscles can develop to help support long and fast efforts.
So, how fast or slow should you run? Depending on where you look, there's different opinions; however, most marathon training plans encourage nearly all of your running miles--70 or 80% of your total week's miles--to be run at an easy effort. You can gauge an easy effort if you have a heart rate monitor, and if you know your maximum heart rate. Generally easy runs should be no more than 80% of your maximum heart rate.
An even easier way to assess how easy or fast to run is by feel. You can use the "Happy Birthday" test. If you can easily sing the song while running, that's an easy effort. If you're struggling to sing the song without giant breaths, that might be a moderate effort. And if you can barely squeak out the words at the end of your 3.5 miles, that would be a hard effort. If an 8:00 mile is your 5K pace effort, then an easy effort could be as much as 1:30-2 minutes slower.
As a former D1 basketball player who's most likely accustomed to doing all of their running at anaerobic efforts, it may seem like slow efforts are just "junk miles." Sometimes I still think that way, and I've run several marathons!
But then I think about Eluid Kipchoge, who is the world record holder in the marathon, and the first person to break the 2-hour marathon barrier. When he set the world record in the Berlin Marathon, he ran most of his mile splits around 4:30-4:39, which is blistering fast. Yet many of his training miles are done at an 8 minute pace. So if the fastest long distance runner alive can run slow, I figure I can too. And so can you!
I hope these resources help. And we're looking forward to helping you when it's time to run that first marathon! Enjoy the miles!