Just say you were in business for yourself, managing a freelance quality assurance consulting firm. This is basically what you have been doing. Provide the average number of hours worked per week and say you had a variety of clients. The state agency I work for frequently asks job applicants who say they were in business for themselves to provide documentation of income (they ask for copies of your tax returns).
Actually, you don't need to "explain" a 13-year absence from the job market. If you choose to, you might say you were out of the job market raising children, during which time you did substitute teaching, temp work, volunteered, furthered your education, managed a research project, wrote a novel, or any number of things. I interview many applicants at my agency. It's amazing how many have large gaps in their employment history. I really don't care. All I care about is whether they have the education, skills, and work experience to meet the job requirements that are specified in the job description and that they can document that they meet those minimum requirements. If they have additional work history or a big employment gap is irrelevant to me.