Modern synthetic shoes do not require breaking in. (In fact, they often wear out or lose their cushioning after about 500 km.) Your shoes should be comfortable from the first wearing - they will never get more comfy. Some problems with the fit or feel of the shoe will not appear until you have worn them on an all-day walk. Since you are in the US, you could start at REI, which has a generous return policy. Of course they will still only have a limited selection, but it is a good place to start.
Trail runners (not waterproof) are probably the most recommended type of shoe. However, comfort rules everything. I have not found a pair of comfy trail runners in the width I need, so I use regular running shoes.
Here is my routine:
- Late in the day after I've been on my feet for a few hours, I go to the shoe store with a couple of different socks. Check their return policy.
- Try on shoes to find the most comfortable model that seems to suit your feet. Start with you normal size and then try the next size larger and the next size wider if available. Keep going until you get to the size that is definitely too big. Then go back to the last size that is not too big. Try them with a slightly thicker pair of socks. Is there still room?
- Walk around the store for 20 minutes. Use their step/incline device if they have one.
- If they seem good, take the shoes to an indoor shopping mall, since most shoe stores will allow a return if you have not worn the shoes outside. Walk briskly around that mall for 30 minutes or so, getting some speed, good stride, stopping and starting, turning, etc. If any problems arise, you can return the shoes, get a different size, or look at a different model. Sometimes I will go to the mall with two pairs of shoes (different size and/or model) and compare them.
- Now that you think you have good shoes, you need to test them on some long outdoor walks. Start with a few km, and try different socks. If they are good, work up to longer distances. You should try to get 2 or 3 consecutive days of 15-20 km to see how your foot responds. You might develop blisters, or discover vulnerable spots. That might be an issue of your foot, distance, socks or heat, etc., rather than a problem with the shoe. However, it you get a blister on the outside of your baby toe, I would say the shoe is not wide enough. Protective tape works well with certain areas. I apply tape across the ball of my foot and on my big toes every morning on the Camino, since those are spots where my skin seems vulnerable.