The 77-year-old actress plays Rose Buck on the BBC period drama, but suffered a stroke and heart attack before the second series began shooting.
"Everyone seems to have forgotten about it now!" she told What's On TV. "Physically, I was able to walk, I didn't look different, but it just takes time to recover. Now it is lovely to be back at work."
Marsh will now reprise her role in the third and sixth episodes of the new series.
"I'm so proud of [the show]," she revealed. "After leaving what it was, coming back and taking the show into the 1930s was very different and exciting."
Upstairs Downstairs star Neil Jackson recently told Digital Spy that Marsh will likely return as Rose full-time if the show gets a third run.
"Hopefully - God willing - when we get a third series, she'll be back fighting fit and resuming her place," he said.
Upstairs Downstairs continues this Sunday at 9pm on BBC One.
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Watch a clip from last week's Upstairs Downstairs below: