ESCONDIDO -- Just a few days after his 76th birthday, Don Parnell is leading a hike, and invites the fitness-minded public to come along.

The 50-mile progressive hike through Daley Ranch begins Wednesday and continues through Sunday. Starting with eight miles on Wednesday, the hikes get one mile longer per day, adding up to the 50-mile total. Parnell, a naturalist and marathoner, turned 76 on Jan. 5.

Hikes tend to fall into two categories, he said. One favors instruction in natural history, the other emphasizes physical fitness.

"I try to straddle the line," Parnell said, mixing in education with exercise, although his hikes have more of a physical component.

On long hikes such as this week's, Parnell said there's plenty of time for instruction in addition to exercise. "What I say depends on the season, the condition of the flora and the fauna."

Located in the northeast corner of Escondido, Daley Ranch is a conservation area covering 3,058 acres which is managed to preserve its habitat. Its hikes begin at the ranch entrance at La Honda Drive unless otherwise stated. The hikes begin at 8:30 a.m and last 3 1/2 to 5 hours. Detailed information on this and other hikes is posted on the city's Web site at http://tinyurl.com/y48gcb.

Those deciding to tackle the progressive hike should be sure to bring water or another nonalcoholic liquid.

"The only source of water is at the ranch house, and we may only pass the ranch house once during a hike and not always," Parnell said. "I find for me two liters lasts about 15 miles."

Recommended clothing includes a sun hat, hiking boots or treaded shoes. Parnell recommends wearing layers of clothing, with the layer closest to your skin being made of a fabric that wicks away sweat. Cotton does not qualify, he said.

Those who complete all 50 miles of the hike will get a certificate of completion.

Parnell is a veteran of the ranch, not only with hiking, but with running. He's slowed down, but isn't stopping.

"I used to lead one-, two- and three-hour runs at the ranch," Parnell said. "I had been a runner all my life, I ran five marathons a year. With the rocks and hills and terrain, I started to get bone-on-bone pain. One day I was out on the 12-mile hike and had to give up after eight miles."

In addition to getting a total right knee joint replacement, Parnell said he had meniscus repair on his left knee.

The wear-out-not-rust-out aficionado will soon be celebrating another birthday anniversary: That of his artificial right knee, installed last February.

It's working great, he said, and he expects it to last another 14 years. That would bring him up to 90.

Before then, Parnell said he plans to lead many more hikes.

Contact staff writer Bradley J. Fikes at (760) 739-6641 or bfikes@nctimes.com. Comment at nctimes.com.