Very first thing to remember is that you have to come back to organ playing very gradually. You see, even if you were quite accomplished in your early career, many years perhaps have passed without you practicing the organ so naturally you have much less ability right now. So what I recommend for you when you practice organ is to take easier pieces than you played in the past. Spend some time with them and you gradually will be getting in a better shape.
Another thing to remember is that your practice time also has to be quite modest at the beginning. Even though you might have practiced for many hours during the day in your early years, it doesn't mean that today you have the same kind of stamina. So start practicing for small periods of time of about 30 minutes long, take frequent breaks and rest a little. This will help you not to overexert yourself and keep motivated for years to come.
Additionally, you can start working on your finger and pedal technique and hand and feet coordination exercises. I recommend playing manual and pedal scales, arpeggios and other special exercises. These technical exercises will help you to get in a better shape and regain your former organ playing skills.
Finally, remember that you have to have some fun so choose your organ pieces wisely. Play organ compositions that you love and enjoy and never forget that organ practice is a privilege. Whenever you practice these organ compositions, make sure you fix your mistakes, work in small fragments, choose a slow tempo, and practice in separate voices and voice combinations. With time this will help you to advance in organ playing and get back to your solid technical skills.