If you are new to running, the most important thing is regular training in the weeks leading up to the event. You may find the following websites a useful source of information

You may want to join a local running group.  Hazlemere Runners is very local, very friendly, and ideal for beginners.  Or try a local running club: Handy Cross Runners; Wycombe Phoenix; Chiltern Harriers; Running Sisters; they will all be glad to welcome new members.

Have you tried Parkrun This is a free 5km run every Saturday morning at 9am.  The nearest one is at Wycombe Ryewith the start and finish by Wycombe Rye Lido, off Bassetsbury Lane, High Wycombe, HP11 1QX.  There is also a parkrun at Black Park (between Gerrards Cross and Slough).  You must pre-register online before you attend your first event, but after that you can just turn up and run any Saturday at any parkrun when you feel like it.