Like Raw Hide said, start close during your daily shooting sessions and progress backward from there. Then to gain strength, here is what I did: Every other night before you go to bed, just close your eyes, draw your bow and hold it at anchor for as long as you can. Then let it down slowly, rest a minute, and repeat. CONCENTRATE ON GOOD FORM! Pull-ups may help you get in overall shape, but they work the back muscles in the wrong direction. Try seated rows or lawn mower pulls with a dumbell. Also do push ups to strengthen and stabilize your bow arm.
Matt