Howard Weitz is a cardiologist who carefully avoided elevating his own heart rate for more than 40 years.

Until recently, the last time the Elkins Park resident remembers working out was in a mandatory high school gym class.

"I'm a lifelong denier of exercise," said Weitz, 64, director of the Division of Cardiology at Jefferson University.

Yet on Sept. 18, Weitz was one of more than 500 sixtysomething runners to compete in the Rock 'n' Roll Philadelphia Half Marathon.

"That sense of accomplishment after doing the half . . . it's indescribable," Weitz said.

While an annual report by Running USA lists the median age of half-marathon finishers at 39, races draw plenty of athletes who find their running legs later in life, and the trend is growing.

Last weekend, 315 sixtysomethings signed up for the Philadelphia Half Marathon, and 254 for the full.

And while many of these super seniors have always led active lifestyles, Weitz represents a new generation of athletes who are finding that it's never too late to jump on the bandwagon.

'I can do this, too'

In October 2015, Weitz found himself at the finish line of the Rock 'n' Roll Half to cheer for a friend.

What really struck him were not the fastest runners, but those at the back of the pack.

"I saw so many unhealthy-looking people finishing and I thought, I can do this, too," Weitz said.

The next day, Weitz walked into Jefferson's gym, sought out a personal trainer, and explained that he planned to run a half marathon, and he had 10 months to get ready.

Was that nuts?

Not at all, says Michael Ross, a sports medicine physician at Rothman Institute. For someone Weitz's age, Ross advises a more cautious approach than for a younger person.

"But there's probably more benefits to starting a training plan at that age than there are risks," Ross said.

For older adults going into training, Ross recommends getting a cardiac stress test, to rule out the risk of exacerbating any underlying heart problems.

"Plus, there are certain things that change with aging - the lungs become stiffer, the muscles lose their flexibility," Ross said.

Weitz didn't do all that, but knew that reaching his goal would take time, something a busy cardiologist might lack.

Weitz figured it out.

"Everyone thinks they're too busy to train for this," he said. "For me, the way to make it work was to come in early in the morning and use the gym facilities at Jefferson before my day started."

Weitz ran on the treadmill three times a week, and then ran outside on the weekends. At first, running one block left him winded. But he persisted, basing his pace - about 4 miles per hour (or 15 minutes per mile) - on his heart rate, which he aimed to keep under 140.

Three months into his training, Weitz grew frustrated with his slow improvement.

"I hit a point where I was getting really short of breath going above 4 mph," Weitz recalled.

He knew he needed expert advice.

Increasing his pace

Weitz scheduled a session with Ross at Rothman's Performance Lab in Bryn Mawr to find out how he could increase his pace.

"It changed everything," Weitz said.

During the hour-long consultation, Ross put Weitz through a cardiopulmonary exercise test.

"I look at the interactions between the heart, lungs and muscles, as well as your running form," Ross explained.

Patients wear a tight-fitting mask and run on a treadmill to measure the amount of oxygen their body uses and the amount of carbon dioxide they exhale.

"This helps you find the point at which your exercise becomes anaerobic - as in, you're making more waste products than your body can get rid of," Weitz explained.

Ross found that Weitz reached his peak performance at about 5.2 miles per hour (11:30 minutes per mile).

Based on the test results, Ross designed three training zones. For the next six weeks, Weitz would try shorter bursts of higher-intensity exercise rather than running at a flat pace for extended periods of time.

The easy zone

For the easy zone, Ross recommended a run at 5 mph for 45 minutes to an hour. A harder zone had Weitz running at 6 mph (10 minutes per mile) for 4 or 5 minutes, followed by 1 or 2 minutes of walking. These intervals were repeated six to eight times. The extreme zone - 7 mph for 22-second intervals - was too fast for Weitz.

Weitz followed the 6 mph intervals for three sessions during the week and on the weekends he completed one longer run at 5 mph.

By week five, the new training regimen paid off.

"The intervals were really challenging but that's where I started to notice a dramatic increase in my functional capacity," Weitz said.

On this program, Weitz was able to keep his long-distance running to a minimum, maxing out at nine miles.

"When I started out, my cruising speed was 4 mph so I could run at that pace for forever and I'd be fine," Weitz recalled. "After seeing Dr. Ross, it's now closer to 6 mph."

The benefits of Weitz's new running program have extended into his work, helping him to better advise patients.

"I'm really practicing what I preach," Weitz said.

Before, he based his exercise advice solely on the medical literature he studied. Now he can also draw on his own experiences.

"It's led to a richer, more personalized interaction," Weitz said.

On race day, Weitz completed the half in 2:48:06 - 12 minutes faster than he anticipated - finishing alongside his daughter and son.

"I really look forward to running with them again in the future," Weitz said.