Evidence can include screenshots of photos of running tracker apps on your phone, your smart watch, a photo of a result from a leaderboard from another race you have done (with your name) a photo of you with your medal from the race or a parkrun time.
You can submit evidence in parts, for example for a 10 mile challenge, you could submit 3 images, one of 4 miles and two of 3 miles. Please make sure it adds up to the right distance, and use google if necessary to translate from kilometres to miles!