It is a noble goal, that's for sure. Running is a different animal than cycling so be careful.
I only run twice a week, 5-6 miles tops, and the reason is that my body doesn't really tolerate it, especially my knees. I'm only 33 and somewhere along the line I've accumulated a lot of wear and tear on my lower body. For me a century is nothing. Running 26 miles, on the other hand, is possible but probably not worth the pain and expenditures.
I know there are some excellent books on running methodology and training. I'd read up if I were you and form a detailed plan to reach this goal. Also, get the right equipment in terms of shoes and inserts to correct any biomechanical problems you might naturally have.
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