The children of Dr. Philip Thompson, a retired rheumatologist who celebrated his 100th birthday last weekend, have donated $25,000 in his honor to the “Close the Gap” campaign of the Eastern Trail Alliance.

The alliance is raising $3.8 million to complete a 1.6-mile, off-road section of the recreational trail in Scarborough, including bridges over the Nonesuch River near Eastern Road and railroad tracks near Pleasant Hill Road.

Thompson, who lived in South Portland for many years before returning to his native Portland, is a charter member of Portland Trails and a longtime supporter of recreational trail development. He said his family’s decision to support the “Close the Gap” campaign reflects his belief that increasing outdoor recreational opportunities can save lives.

“Exercise improves your health and extends your life,” Thompson said. “It’s life-saving.”

The Eastern Trail is a 65-mile on- and off-road recreational trail that runs from South Portland to Kittery. It’s part of a larger effort to establish a 3,000-mile East Coast Greenway from Calais to Key West, Florida.

The alliance has raised $3.25 million through the “Close the Gap” campaign and plans to complete the project in 2018, said Executive Director Carole Brush. Then, trail users will have access to 16 contiguous, off-road miles from downtown Saco to Bug Light Park in South Portland.

“We are very grateful to the Thompson family for their generous contribution,” said Deborah Erickson-Iron, alliance president. “To get the trail completed is going to take significant donations like this.”

Since 2012, the campaign has received contributions from:

• The Maine Department of Transportation, $1.55 million.

• The Portland Area Comprehensive Transportation System, $1.1 million.

• Scarborough and South Portland, $286,000.

Town and Country Federal Credit Union, $100,000.

• Avangrid, $25,000.

• The Rugged Alliance Foundation, $5,000.

The inaugural John Andrews Memorial Eastern Trail 5K will be held at 9 a.m. Saturday to benefit the campaign. Named for the alliance’s founder, who died in February, the walk/race will start at O’Reilly’s Cure Restaurant & Bar in Scarborough. Registration costs $20 online at www.active.com until 8 p.m. Friday or $25 at the race, starting at 7:30 a.m. Saturday.