Good for you. It's a worthy goal. 30 miles walking per day is really achievable with enough training. I ran a 50k in 2015 in 6 hours that was around Spokane and largely level. My trail runs in the Kettle crest were 34 miles in 11.5hrs and 35 miles in 9.25hrs. You just need to find or build a solid training plan and stick to it as much as possible. My training impetus was maintaining a daily streak which by default got me in great shape, too bad I broke the streak with injuries and couldn't get myself back to training.