Back in the days of the old Emerald Coast Racing Team, the "what if" or "what now" conversation was common on the day after a race. At least during the first miles. One of the guys, Gary, would say, "the day after a bad race I run easy; the day after a good race I run easy." Can't say the runs were all that easy back then. It was a historical inevitability the ECRT crew would run relaxed for perhaps a mile, at which point the hammer-fest would begin. Former CU cross-country runner-turned-running coach Jay Johnson (immortalized in one of the recent classics of running literature, Chris Lear's Running With The Buffaloes) would most likely have a sense of deja vu were he to see that group.
The other question which comes up during that time when euphoria and (oral) anesthesia are at its peak, or,during the "what's the next target" conversation as we climb a bridge or rise in the road is "could I qualify for the Boston Marathon at this particular state of fitness?" There's much speculation: Properly training to race a marathon without injuring one's self is more difficult than executing the race on the day. Please note the use of the term "race;" I have friends who have completed a half-dozen or more marathons in a single year, on training even they would say resembled something, er, "pulled from their body."
(I've had these conversations via e-mail in the past. They're not pretty. Usually they involve me screaming at my computer terminal in disbelief.)
It doesn't mean it wasn't physically challenging for them. But the terms they used to describe their experience were closer to "participation" and "completion" than to out-and-out racing to beat as many persons as humanly possible.
The most-modern method of answering this question involves the use of one of many on-line training and racing calculators, one of my favorites is on coach Greg McMillan's McMillanRunning.com web site. Plug in the best or most recent race performance, as well as the target event, and the calculator will provide performance predictions for intermediate distances from one mile up to the marathon distance. Another link will show the ideal pace ranges to enhance running endurance, strength and speed.
On the other hand, if you are "old school" and desire a chart or two to guide your planning, there's nothing better than Dr. Jack Daniels VDOT tables (published in several on-line coaches' education web sites, developed as macros within training spreadsheets and published in multiple editions of his Running Formula, and the book written with Jimmy Gilbert, Oxygen Power).
Two things I like about Daniels' VDOT tables: The recent/best performance data provide a strong ballpark figure of a runner's VO2max score...which quantifies the amount of oxygen a runner can utilize in terms of liters per kilogram of body weight at near-maximal effort. For citizen-athletes it's a way to measure raw improvement in ability. Of course, a high VDOT score and $2.25 will get you a cup of coffee at Denny's in my hometown.
In the same manner as McMillan's on-line calculator, the Daniels' VDOT tables provide training paces you ought to be running at to improve running economy, VO2max, and endurance based on a recent or recent best performance. Earlier versions of the tables provided hard and fast target paces for every type of training, whereas the latest (fourth?) edition of the book provides more flexible pace ranges. The fourth edition of Running Formula also provides three different training plans for the marathon based on performance experience, as well as half marathon, 15, 10, 5-kilometer and 1,500 meter racing plans.
N.B. I tend to ballpark paces for my athletes because they're training more on the roads and tracks but push for a narrow pace for myself.since I use a treadmill more often than not.
Every so often an athlete has a performance at a distance which isn't in the calculators, or their performance falls somewhere in the middle ground between VDOT points. What is a runner to do then?
In these situations a runner can take a known time and extrapolate to "what if" a longer or shorter distance by either adding (for longer distances) or taking away (for shorter distances) a factor of five percent. The McMillan tables can do all that good stuff for you, but if you want to play with your pocket calculator one more time to see if you still remember high school algebra...go right ahead.
In the case of a runner who just recently ran a 10-kilometer on the roads and wants to know their projected marathon time, Marathon Time = (10K time x 4.2175) x 1.05.
If the 10K was 53 minutes, 33 seconds...
Marathon Time = (53.55 min x 4.2175) x 1.05
Marathon Time = (225.85 minutes x 1.05)
Marathon Time = 237.14 minutes
Marathon Time = 3hr 57min 09sec
With the identical conditions and proper training, a person who runs a 53:33 10K would probably be able to finish a marathon in the 3:57-4:10 range, most likely closer to the faster end, since that 4:10 was based off the Daniels' VDOT tables for the slower VDOT calculation. I like to think the athlete will work hard enough to improve between now and the date of the marathon.