WASHINGTON—Once a simple pleasure, a walk outdoors for Bernie Sanders and his wife Jane is now a complicated logistical challenge with an entourage in tow.

A Secret Service detail accompanies the couple everywhere, and finding time between frenetic rounds of flights, rallies and phone calls is no small feat.

“When there is a break in the schedule, he’ll bolt for a walk,” Mrs. Sanders said in an interview. “It was in the low teens in Wisconsin (on a recent campaign stop) and we said, ‘Hey, we can either eat lunch or go for a walk.’ So we went for a walk.”

Not so long ago the Sanders campaign had the feel of a startup inching its way toward relevance. Nearly a year after launching, it has evolved into a more professionalized operation that looks and feels like a modern presidential campaign, with all the complications that entails. Money is pouring in—and rushing out the door.

After holding out for months following his April 2015 entry into the primary race, Mr. Sanders agreed over the summer to hire a polling expert—although he didn’t start paying for surveys until the fall. A voter modeling firm that worked for Barack Obama’s 2008 campaign now helps the campaign reach targeted voters.