Organisers of the City-Bay Fun Run are re-evaluating the event's future, following a significant drop in numbers and increased competition from similar events.

About 26,000 people are taking part in a three, six or 12-kilometre run or walk between Adelaide's CBD and Glenelg today — an annual event that started in 1973 and had a record 40,000 entries only a few years ago.

The 2017 City to Bay is expected to raise about $400,000 for various charities.

Race director Joe Stevens said it was sure to be an enjoyable day for competitors, but he said he was disappointed with entry numbers.

"It's our 45th year and we'd hoped to have it back up to [40,000]," he said.

"But we take what we can get.

"You've got so many events on now … nearly every weekend there's a community or charity event involving running or something else.

"We've sort of got to the stage where we're competing against a heap of other events."

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Today's winner was Australian Olympian Brett Robinson, who finished in under 36 minutes.

The first women over the finish line was fellow Olympian Jess Trengrove, from SA, who completed the run in under 40 minutes.

Mr Stevens said the event would now be reassessed, though he said it was too early to know what kinds of changes — if any — would be made.

"We're going to [reassess] it now … we thought that the Bay-City, the reverse event … may have taken people away from [the City-Bay]," he said.

"So we're just going to [re-evaluate] it now with all our events that we do."

The Bay to City event, which is held annually during March, began in 2015.