For the record number of American runners who completed an official race event last year, the questions often start not long after they cross the finish line: 'What's my next challenge?' and 'How much further can I push myself?'

But data show that the bulk of runners are heavily concentrated in shorter races, specifically 5 and 10 kilometers. That could be because many who aspire to race longer distances struggle with how to successfully ramp up training and stay motivated.

Of the 15.5 million race finishers in 2012, 40% completed a 5k (or 3.1-mile) race, while 10% finished a full 10k, according to Running USA, an industry nonprofit organization based in Colorado Springs, Colo. By contrast, only 3% accomplished a full marathon.

'Going from 3 miles to 6 miles -- a 5k to 10k -- that's logical. Going from 6 miles to 26 miles is a whole different world of stress,' says Tom McGlynn, a three-time Olympic marathon trials qualifier and founder of Runcoach, an online training program for runners. Longer distances require a much different mind-set and approach than shorter ones, he says, with more focus needed on muscle stamina and endurance.

Doctors and running coaches advise beginners to transition slowly. Some recommendrunningregularly at a 5k to 10k level for six months to a year before training for a marathon, while others suggest dedicating one to two years to building a base.

All agree -- don't wait too long after a first race to start working on the next one.

Mr. McGlynn says he advises runners to sign up for a second race well in advance of their first 5k or 10k. That way, he says, they immediately have another goal to train for.

'A 10k doesn't take a lot of rest. A day or so and you can get back into training,' says Bill Roberts, director of the University of Minnesota St. John's Hospital Family Medicine Residency and medicaldirector for the Twin Cities Marathon.

Generally, runners shouldn't increase their mileage by more than 10% each week and should run three to four times per week, Dr. Roberts says. One of those runs should be longer and paced more slowly to help runners increase their endurance and stamina. 'If you're a new runner, it takes a long time to toughen the tissues,' he says.

One problem runners face is a lack of interim-length race opportunities. Last year, there were roughly 60 12ks and 320 15ks -- a fraction compared with 3,200 10ks and 1,900 half-marathons, according to Running USA.

Boredom and mental burnout can also trip up beginners who are moving to longer distances, says Jennifer Van Allen, a running coach and co-author of 'Runner's World Big Book of Marathon and Half-Marathon Training.' 'Go with a buddy, go with a group, run with music, run without, run with a watch, run on trails or run on the road,' she says.

Half-marathon and marathon training plans also introduce a new component to the weekly schedule: the long run. These typically start with 5 or 6 miles and build up to near-race distance by the last quarter of training, says John Honerkamp, manager of runner products and services and coach at New York Road Runners.

'For a beginner, the long runs each week are pretty daunting,' Mr. Honerkamp says. 'Focus on a sign, focus on the people you're running with,' he says, which will help combat monotony. Another strategy: Dedicate each mile to a friend or family member. Or if it is windy, think of pushing against the wind. 'It might seem silly, but if you're on mile 22, you need these mind tricks,' Mr. Honerkamp says.

Overweight and older runners should check with their doctor before starting a training plan. 'If you're overweight, it is probably good to spend time working on the weight loss and strength to support that weight, and bring that weight down before you increase to a half-marathon or 10k,' Dr. Roberts says.

He also suggests alternating strength days and running days. Runners should focus on their core, including their abdominals, hips, glutes and lower back, as well as their upper body, he adds.

Once the regimen reaches about 30 miles per week consistently, 'you're ready to make that next jump to marathon distance,' he says.

Meg Navatto of Oceanside, N.Y., decided to start running about three years ago to lose weight. 'I was very heavy, over 200 pounds, and I'm only 5'2.' I couldn't even walk up a flight of stairs without huffing or puffing,' says Ms. Navatto, 38, a cytogenetic technologist. When her husband Pete, also 38 and a New York State Court officer, said he wanted to start running, she offered to try.

They ran their first 5k in September 2011. He came in second place in his division. She came in last -- but both were hooked.

They continued to run three days a week after work for 30 minutes. 'We'd go to the park in Valley Stream near where we live. We'd loop in opposite directions so we ran at the same time, but not the same pace,' Mr. Navatto says. After Mr. Navatto completed a half-marathon and Ms. Navatto finished a 10k in May 2012, they started adding in one long run on the weekends.

Now, they are training together for the New York City Marathon in November, following a 20-week customized program from New York Road Runners. They typically run five days during the week, at varying speeds and terrain. They run an organized race pretty much every weekend, in addition to a long run.