Hi Alyson!
This is a great goal! Turning 40 is a big milestone and I think this is the perfect way to celebrate!
I started running for the first time when I was 32. I NEVER, EVER, EVER thought I would run farther than a 5K. I completed
my first marathon this year, so I know you can do it, too!
My biggest advice is to check out the
Jeff Galloway method. His run/walk/run method has helped so many people. The walk breaks are as good for my mental health as the physical aspect. Breaking your run up into smaller increments is amazing! You can read about him on the Disney website, and also by visiting his page (www.jeffgalloway.com). He has great plans for all levels of runners.
If you have a buddy that could join you on some of your training runs, that is a great idea, too. It's always easier to do things when you have someone there to motivate you, and you can be there to motivate them! Also, find a local 5K to sign up for in the late summer. Having a race on your schedule can keep you in line, and the race experience is always a great motivator. Crossing a finish line is very special and you will want to do it again and again!
I hope this helps. Please keep in touch and let me know how your training is going, and let me know if you have any race specific questions!