The short answer is build something people want. It’s really good advice from YC, and applies whatever your goals are (big software company or smaller niche businesses).
I’d just focus on finding problems and solving them, while getting people to pay to use them. Don’t overthink it!
You could also look for some part time contracting work in the meantime to keep an income and stay in touch with a professional network.
Congrats on the weight loss and fitness, that’s huge.