Location is important.
I worked at a tennis specialty shop that expanded over the years. The shop started out near the city tennis courts--perfect for its target market.
Owner slowly moved into running and then aerobics. Stuck primarily with shoes. Clothing is a pain because it is seasonal and trendy--especially the aerobics clothing.
Getting suppliers to issue credit to give you 60 days is key. Back in the old days, the owner would have to write a check for COD when deliveries came. That is a cash flow nightmare.
Someone above asks about suppliers. I think some companies grant territorial rights. So ASICS (not sure it does it) might say that only Store Z can sell ASICS within a 35 mile radius. I have wonder about the legality of it as well.
I would look at a little diversity. At the store I worked at, our biggest sales area was really sneakers for everyday wear more than specialty running shoes. Back to school shoes and Xmas was probably 2/3 of annual sales.
At the same time the store stayed with what it knew: good pricing, good service.
Good luck if you choose to pursue. I hope you have good capitalization to make it. Too often it seems people get in with little money in reserve and have no cushion.