The 5 km races and run/walk events offer fair weather exercisers the chance to support a favorite charity without too much wear and tear on out-of-shape joints, said experts.

But experts say for anyone who really wants to kick-start a fitness regimen, lose weight, or just change their sedentary ways, training for a 5km run may be the shortest distance from couch potato to easy runner.

'It's a better first step than just setting a goal to lose weight,' said Matt Fitzgerald, a coach and editor at Competitor magazine.

'Human beings are natural game players: we like having a goal, a game plan.  There's no game in losing 4.5 kg.'

And there's no glory in staring at a scale. 'The magic of crossing the finish line is what hooks people,' said Fitzgerald, who is based in San Diego, California. 'It's life changing.'

Thanks in part to charity events, the little race has come to dominate the field. Some 4 million people nationwide crossed a 5km finish line in 2009, according to Running USA, a non-profit organization for the running industry.

'Event organizers push it because of the accessibility of the distance. Marathons are not for everyone. The 5km is a lot easier to host,' Fitzgerald said, and a healthy adult can get in shape for a 5km within 10 weeks.

'Most people can walk a 5km right now,' he said. 'Just try to get to a point where you can jog the whole way.'     Pete McCall, an exercise physiologist with the non-profit American Council on Exercise, said a top distance runner can knock out a 5km in 14 or 15 minutes. 'For beginners 30 minutes is a moderate and accessible target.'

But be prepared to walk before you run. 'Walk three-to-five minutes, then run three-to-five minutes, then gradually increase the amount of time you run,' McCall said. 'It's all about setting goals and giving your body time to adapt.'

He said any realistic plan should also involve flexibility and at least three days per week of core-stabilizing exercises. To avoid overuse injuries, runners refrain from doing too much too quickly.

McCall recommends that the novice invest in a shoe designed specifically for running. 'A running shoe has a different design and structure than a training shoe, which is wider with a different type of foot bed.'

Both the ACE and Competitor offer the beginner 5K training tips and plans. McCall said anyone aiming for a more challenging race should find a trainer.

"From what I've seen the 5K gets people more involved in running," he said. "You establish a goal, and train for it. When you see your time drop from, 30 minutes to 28, you say, 'Yes. I can do this.'

He said even if a person's sights are set on the marathon, the first goal should be the 5K.

Fitzgerald said while some use the 5K as a stepping stone to longer races, others stick with the short and speedy.

"Marathons are really not for everyone," said Fitzgerald, who has run 14 of them. "Some people just get so excited that they jump straight up to half marathon, but no one should feel compelled."

He said many find their niche in the 5K."After all, you can go to the Olympics with 5000 meters," he added.-Reuters