Jenny,
If you plan on using the new shoes for the race, make sure you break them in first. I'm sure some of the more experienced runners can tell you how to do that best.
I bought my shoes about a month ago. The old ones weren't bad, but they got tight after about 9 miles. I am really enjoying my new pair!
If your shoes are fitted properly and you have the correct shoes you should be good to go with 5-15 miles on the shoes pre-race. That does not mean wearing them to the airport and race start and calling those miles.
If you get shoes for Christmas you should be fine if you take them out for a few spins.
First you should wear them for an hour or so indoors just to make sure you like them - assuming new style from your old style or any other change such as size. Then take them on a 3-6 mile run/walk to see how they feel. Keep it close to home and keep thinking about how your feet feel. Any hot spots or slipping that may need attention. Take a couple of these length runs then get out for at least one long run in the shoes pre-race. For a half I would try to get 6-8 miles under them then put them up for the race.
More Coaching to follow
Keep a log of you miles on your shoes! I can almost guarantee that your shoes need replaced well before you look at them and I need new shoes. For most of us shoes only last 300 500 miles. If weight challenged or if you have a structural issue, you will only get the lower end of the range.
If you have not been fitted in a running store get to a running store not a box store like
WalMart of Dicks Sporting goods as the person in the shoe department may have been selling fishing gear yesterday. You will be sized, your feet evaluated for arch type and given 2+ pairs of shoes to try. Take them for a quick little spin in the store, out the door or on the treadmill; different stores have different policies. You want to have the sales person look at your gait to make sure they have you in the correct type of shoe cushion, stability, motion control. Only if you are split between two or more shoes should styling come into play.
When you get your shoes home take a Sharpie and make a code on the shoe so you will know what pair you have on. I use purchase date (mm-yy) on the inner side of the shoe sole. Then break them in as above.
Never wear your running/walking shoes for anything other than running/walking. Dont wear them to the store or out for a night; only for training and racing. Log the miles as you train so you can keep track of the shoe life. Never wear the shoes for training two days straight (another reason to wear them only for running). The foams used in the shoes need time to recover just like the muscle fibers in your legs. They need to dry out also; else folks will wonder where the teenage boy is when you walk into a room.
A helpful hint is to purchase two pairs of shoes and rotate them. I have about 150 miles difference between both pairs of shoes at any one time. That allows me to make a style change to see if I like it while only investing in one shoe from the new style. Having the 150 mile difference between shoes also give you a gage of whether the older shoe is nearing its end. If the old shoe feels too hard, or you find yourself skipping the use and staying in the new shoe, it is time to retire the old shoe.
Hope this helps and sorry if I got long winded