A 4000-metre flat race at Sandown on Wednesday will not only be a distance test for stayer Luck's A Fortune, but also his jockey.

Apprentice Harry Coffey has ridden the Pat Carey-trained Luck's A Fortune to back-to-back wins in both heats of the Marathon Series at Sandown over 3200m and 3600m and has the job again in Wednesday's Marathon Series Final.

"He actually was a bit keener last start than the start before, so it was actually a bit more tiring," Coffey said.

"So hopefully I'm fit enough for the 4000 metres. That's my only worry, if I can get the trip. Because he'll definitely get it."

Coffey suffers from Cystic Fibrosis but hasn't let that stop him from achieving on the racetrack during his apprenticeship.

He has ridden 50 winners so far this season in Victoria including 17 in town and has notched metropolitan doubles twice in the past month.

Luck's A Fortune, winner by more than four lengths last start, is one of six rides for Coffey at Sandown.

"He has improved the further he has gone, so hopefully he improves again," Coffey said.

"I think I'm on the right horse."

Riding over a long distance will be a new experience for some of the jockeys.

Three-time Melbourne Cup winner Glen Boss rides By His Design while Craig Newitt, who has won a Sydney Cup over 3200m, partners Hawkesinthesky in the nine-horse field.

Coffey sits third in the Melbourne apprentices' premiership, one win behind Katelyn Mallyon and six behind Patrick Moloney who is sidelined for an expected three months with ankle injuries suffered in a fall.

The Sandown meeting features the return to race riding for Dwayne Dunn.

Dunn has not ridden since mid-December after being diagnosed with bulging disks in his neck.

The jockey, who finished second in last season's Melbourne premiership, has three Sandown rides.