Thank you for your question You may find that you can increase the duration of erection by practicing pelvic floor exercises and some specific masturbatory exercises. Whether this can be 20 mins is hard to say. You will have to try them and see. Read 'New male sexuality'  by Bernie Zilbergeld. There are no medicines to increase penis size but you can learn to give more pleasure through reading or watching DVDs on the subject. In the end its what's pleasurable to your partner that matters in sex. So find out what gets her going n you will win every time. Read ' How to make great love to a woman' by Philip Hodgson hope this helps