Train with a large dildo to stretch everything a bit and there is really no other problem than the usual "exercise sourness" feeling a day after a BBC and that is just the same sourness you feel in any muscle that have been trained, so hard to remove that really, the only way is to fuck more
Also it gets better by itself after awhile, your body needs time to train the muscles that lifts the cervix so his cock can go up into your posterior fornix. Its extra space that many girls dont know they have... and it feels great when a guy reaches into there! Its the "g-spot" only size queens and bbc sluts know about