Great job there. I rode 15 miles my first time out and my tush was a bit tender. A little over a month later and Im up to 70 miles once a week. Keep riding and if you have trouble with the soreness, look into a good saddle, riding shorts or some combination of both. Also remember that its not a bad idea to have shorts with different chamois if you get them so they may pressure you in a slightly different way, and most of all, keep riding and have fun. You may find twenty minutes a day wont satisfy your riding lust after a time though, and you will be trying to find ways to fit more riding time in.