I luckily have my husband to ride with, altho its rather frequent that he will leave me high and dry, all by myself, pushing along at my own pace (needless to say his legs are much stronger than mine so he leaves me in the dust). But what I found works for me, is that when I have crested the 12 mile mark and feel that I just can't go anymore, I slip into an easier gear and remind myself that I am not a quitter (I started at about 9 miles in an hour's time 6 weeks ago and now am about 14 miles in an hour's time). Its like I get my second wind and after that slight breather on that easy gear, I'm changing it back up and adding distance beyond my initial goal. Some nights are better than others, but I have progressed quickly, and you will too, before you even realize its happened.