Training for 25K
If you have run a half marathon in the last 12 months it should be fairly easy for you to train and run the 26K race. The training program for a 25K race can pretty much be the same as that for a half marathon except that the longest train run can be 24Km (15 miles) in stead of the usual 12 or 13 miles.
If you have not run a half marathon before and the longest distance you have ever run is 5K, you can still train to run the 25K race over the next 3 months. Even if you don't run, but are reasonably fit, you can walk the 25k comfortably, since the course will be open for 10 hours i.e. till 6:00 PM.
There are several Half Marathon training programs available on the Internet. Here are links to a few of the popular ones: