Life Changing Decisions

Options
Hello everyone. This is my 2nd time using this app, I used it about 18 months ago and lost over a stone in weight. I lost focus but I restarted about a month ago.
I now run 5K every other day in less than 30 minutes which is quite an achievement for me. I've lost 3lbs and I'm determined to keep going. I intend to join a gym to do some weight training as well.
Has anyone got any advice for me in putting together a muscle gain programme?
What keeps your motivation once you reach your weight loss goal.
All advice greatly received.