The handicappers did a good job as 23 of the contestants finished within a minute of the winner.
The field was divided into four handicapped groups, with Walker and McDonald first away as part of the 10-strong limit bunch.
MacDonald and the evergreen Laurence Sunde pushed the pace, shedding riders along the way, as the number whittled down to four shortly after the halfway turnaround.
Accompanying Sunde and MacDonald were Walker and Pania Hudson, both riding well and contributing to the pacemaking when they could.
Behind them, a hardworking second break group of six riders combined well as they chased the leaders, although they were down to five before the turn as Martin Hogan and Ross McNeil kept the pressure on.
Graeme Cox, Pat Carroll and Dan Richards were also there as the four minute starting gap between the groups was halved.
Meanwhile the front groups were themselves under threat as nine break riders came up to hunt them.
Sean Tucker was pouring on the horsepower while Allan Luff, Darcy Forrester and the teenage pair of Lewis Bunker and Boaz Darby were all looking particularly strong.
The six scratch riders starting at the back were also putting in a big effort, but despite the combined ability of Adam Gosney, Brent Bismark, Adrian Pettit, Bevan Cheatley, Pat Johnstone and Steve Cowan, they were struggling to peg back the break group.
At Upokongaro on the return leg, the second break had the leaders in sight and were closing steadily with McNeil and Hogan ramping up the pace, with the Tucker group not far behind.
However, MacDonald was continuing to drive the lead group and they still had a slim margin as they entered the finishing straight.
Walker unleashed a fine burst which saw her break clear and withstand MacDonald's late charge to take a narrow win.
Her victory was further confirmation of steady progression over the past season.
For MacDonald , with considerably more miles on the clock, the trophy was well earned as his recent good form reflects his additional training efforts.
Sunde continues to defy time and his third placing was testament to his durability while Hudson put in a great ride to nail down fourth spot just holding out the powerful finishes of McNeill and Hogan from the chasing group.
Carroll was a solid seventh from Cox who was right there in eighth. Right on their tail was Tucker's group Darby and Bunker continuing their weekly battle and sprinting home in that order.
There was a short gap back to the scratch group with Gosney heading them while clocking the fastest time of 1:04.15, ahead of Bismark.
In the shorter, mass-start C-grade race over the same course, Dean Blackwell won with authority riding clear of his eight rivals for a 39 second win.
Debbie Cain battled on strongly as usual to finish a clear second place.
Cherie Prince filled third berth, with a gap to Stefan Bourke, Paul Darby and Joseph Loveridge-Mills.
Results
Open Grade Handicap: 1. Michaela Walker (U17); 2. Murray MacDonald (Winner of Ross Cup); 3. Laurence Sunde; Fastest time: Adam Gosney (52:15.51).
C Grade: 1. Dean Blackwell; 2. Debbie Cain; 3. Cherie Prince.
