Milestones are great to have, and this is a way for you and your equine partner, and maybe even a similarly paced friend, to challenge yourselves to do something bigger. Pace yourselves to accomplish the true goal, and find out what you are capable of. I recommend something along the lines of 25 miles day 1 + 15 miles day 2 + 10 miles day 3 (for example to accomplish a 3-day 50 mile challenge,) but you can do whatever works for you! Maybe you ride 15 miles in the morning and another 10 that evening. As long as your mileage adds up to your final goal after 3 consecutive days, going whatever pace is healthy and fun for both you and your horse.