You cannot expect to do well or to feel well if you immediately go from 3 miles to 6.2 miles. You can post your plan on the refrigerator to keep it in a special file in your computer, whatever works for you. I frequently tell people who are trying to alter behavior, that there really is no such thing as willpower, but that willpower is simply good planning. People continue with a behavior when it's rewarding. Rewards can be either internal or external.
Internal rewards are truly the best kind.
Frequently we learn to be very hard on ourselves and deny the things we most want to hear. When you are trying to obtain those running goals, talk to yourself the way you would talk to and encourage your best friend. External rewards help too. Find some things you really like such as rewarding yourself with a soak in the tub after a run or giving yourself 15 more minutes to sleep in the morning. External rewards can also entail setting aside some money to buy something special such as a cassette tape or a new outfit.