Once you've defined your target market segment (for example: long-distance runners), you can use secondary research to determine the size of the segment and whether it's growing, shrinking, or stable.
What do your target customers think? How are they currently meeting their needs?
Read reviews on Amazon or Yelp or see what they are tweeting and facebooking about the competition.
Market Research Reports outline the macro trends that impact your market.
Statista aggregates statistics and studies from market researchers, organizations, specialist publications, and government sources.