You don't find one, you create one. Instead of forming cool short projects, form a long one. Just as an example, lets take running (since you mentioned it). Instead setting a goal like "I want to run 2 miles three times a week", make a goal like "I want to run a marathon by August 2023." Then setup a plan to make that happen, build up each week until you can run longer and longer distances.
Also, realize that focusing on one project will necessarily mean not focusing on most other projects.
You don't find one, you create one. Instead of forming cool short projects, form a long one. Just as an example, lets take running (since you mentioned it). Instead setting a goal like "I want to run 2 miles three times a week", make a goal like "I want to run a marathon by August 2023." Then setup a plan to make that happen, build up each week until you can run longer and longer distances.
Also, realize that focusing on one project will necessarily mean not focusing on most other projects.