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Get the best sports bra you can afford. And if you're busty, buy two (different brands as they all support in different ways) and wear them both at the same time. It's not comfortable, but running is really hard on your chest... there's no point in the rest of you being toned and lovely if you can tuck your girls into your socks!
Good for you!
First off, don't worry about your time the first time out...just complete the race and have fun! Most of all have fun!
There are many different walk to run programs available through the web and maybe in your town. I joined USA Fit running program to train for a 1/2 marathon. It was great! There are organized runs AND you have running buddies that are your speed.
I have found that I am most comfortable running 5 min and walking 1. This works for me for longer distances and I am able to better my time. Some people run 2/1's or 7/1's. Whatever works! The important thing is to enjoy yourself.
For training tips, I think everyone here has covered what you need to know. You're following a structured plan, so that sounds good and you've got run/walk segments in there, so that's good, too. Also, someone said to run longer than 5K during your training, which i also think is important (for psychological reasons if nothing else).
Soon after I met my husband, we ran a 5K together, so I'm kind of partial to the race distance myself. I've toyed with going longer, but I am primarily a cyclist, just run during the winter, so I really don't have time to train for a longer race.
As for the race itself, the best advice I ever heard for making sure you'll have a smile on your face at the finish line: divide the race into 3rds. During mile one, go at your predetermined pace: this should feel slow to you. People will pass you. Let them; you'll catch them later.
During mile two, up your pace until you're breathing hard, but still have something left in the tank. Mile three, go fishing. See a runner ahead of you, reel them in (they probably went out too fast; you might recognize a few of them as the ones who passed you during mile one). The last 0.1 of a mile (since a 5K is 3.1 miles, not 3 miles) is for your sprint finish. Determine where you think you can sprint and hold it across the line and go! This is usually about 200 yards or so.
Most of all, have fun! The 5K is a great distance for getting in shape and can also be used as a stepping stone for greater things ;)