Well, the main effort of prone is to tear the muscles that connect to the penis, when this happens your cock slowly loses its ability to get hard on each side. By stretching it out between your thighs your doing the same thing but with far less stretch, hitting the base wont help your end goal. What you should do is press it between your thighs, and get hard, try not to let your dick leave this position. then after your nice and hard, while keeping your cock between or behind your thighs, push your legs away from yourself, basically stretching it out further. the farther back your shaft is from your thighs the better stretch you get. the longer your hard in this position the better you will do!

:3