There are some pretty good plans to follow to get your distance up. Similar to the C25K plan, there are those that will help you work up from a 5K to a 10K, from a 10K to a half, and so on up to full marathon training plans.
Another thought is to check with local running/shoe stores to see if they offer clinics on running technique and other topics that will help you train and recover properly without injury.
My longest competition run was a 10 miler. I thought I was in shape enough for it. I wasn't, and the last few miles hurt....bad.
Final thought, look into Body Glide or similar anti-chafing cream. You may need it on sensitive areas once you start logging serious miles.
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