Obviously due to the nature of the sport, one would not wake up and decide to run 26.2 miles that morning, because they saw the street closure signs the night before. Almost equally true is the decision to run a 13.1 miler. Most large races (Half or Full marathons don't allow on-site registration anyway) but what they do offer are discounts for registering early. I have heard of people say, "I just decided to sign up!" but I still don't know logistically, how close to the race, they did so. But how early is too early?
If you register 6 months early you can save anywhere from $20-$40 but the registration for a half marathon already is about $60 or more and for a half it is about $80 or more. The small price difference between the two creates an interesting psychological purchase- for only 20 dollars more, I can run twice the distance!- suddenly you have spent an additional $20 and now have to train for 26.2 miles.
When you think of a race that you want to register for, it is important to take into consideration the time available that you have to commit to training and the appropriate expectations for your goals, given that commitment. What kind of family support do you have to train? What do you do for work? Are there slow times in your business, that are good for training? I for one do campaign work among other things, so since elections are either in Nov or July, I expect my busiest months to be Oct/Nov and June/July so I try not to set unrealistic goals during those times.
Next is where will you want to run, this will determine how early you make the decision, because if it is somewhere with different weather or altitudes, you will want to take that into consideration. Not to mention saving money for the trip, or on the trip, when are there hotel vacancies and so forth. These things you will have to discuss with your partner, friends, or coach, whoever accompanies you, if someone does come along. But all of the above, can be taken into account and plans can be made accordingly.
The million dollar question of when to register arises, not because of the things that we know, but rather the unforeseen and the unexpected such as changes at work, injuries, illnesses, lack of preparation. Last year I had a stress fracture, and luckily it didn't fall upon the time that I had scheduled any races, but if it had, I would have been out anywhere from $80- $150.
My rule of thumb to plan a race is to research it, look at the registration schedule/fee increases, and shoot for a decision before the last price increase. To make the final decision I enter a race of about half the goal distance around the same time as that deadline. If I did okay - in relation to what my goals are, or I think I can maintain a schedule to be successful, and I am not waiting for the final date of my Sister's wedding date or something somewhat foreseeable, then I'll register before the last price increase,
For example If I were considering the Santa Barbara International Marathon on Nov 12, 2011, I would pull up this schedule, run a half in June or early July and if I did well, and knew I had time to commit to training for the full, I would register in late July.
$95 Early Bird registration by Feb 28 $115 Mar 1 - July 31 $130 Aug 1 - Nov 2 (or until maximum number of entrants reached).
So far that has worked for me, but I'm not sure that there is a right or wrong answer. What are your thoughts on registering early vs. registering late? How early, is early?