The race had looked a straight shoot-out between Fentara and eventual runner-up Woodford County, but both horses had nothing left to give after the last fence.
Blenheim Brook, conversely, suddenly found another gear when it mattered and the 13-2 shot charged home under Peter Buchanan to win by a neck.
It was an especially sweet moment for Russell, who had broken the Scottish record for winners in a season earlier at Carlisle.
She said: "It's absolutely brilliant. I'm so delighted, not just for me, but for all the owners. We weren't sure he'd definitely stay the trip so it was either going to be the best ride in history or the worst ride in history."
Earlier, Streams Of Whiskey came up trumps after a grandstand finish to the totescoop6 The Millionaire Maker Handicap Chase.
Hollow Blue Sky looked on course for victory after a crisp leap at the last, but the landscape of the race altered dramatically in the final 200 yards.
Forest Walker was firstly bustled into the lead for the Dan Skelton operation, but he was outmuscled by half a length by Streams Of Whiskey, trained in Cumbria by the in-form Nicky Richards.
The Nigel Twiston-Davies-trained duo of Hollow Blue Sky and Frontier Spirit finished third and fourth respectively.
Winning jockey Brian Harding said of the 18-1 chance: "It was a good training performance to get him here in this form."
Richards said: "We've been waiting for this ground. He had run well in a couple of good races against decent horses so he's been a bit unlucky. That's it for the season now."
Jockey Danny Cook was taken to hospital for X-rays on a suspected broken collarbone after he fell from Tiptoeaway.
Maurice Linehan's strength in the saddle helped Carlton Jack (9-1) get home first in the £3 Million totescoop6 Today Handicap Hurdle.
The Jonjo O'Neill-trained seven-year-old gained a commanding lead after the final flight, at which Generous Ransom's error-strewn round of jumping from the front finally took its toll.
Carlton Jack was joined by 5-1 favourite Heath Hunter along the run-in, but the former refused to buckle under intense pressure and was driven out for a length-and-three-quarters verdict.
Linehan said: "The good ground helped him a lot. He doesn't want to be in front too soon.
"He stuck his head out and got me out of trouble."
No Planning survived a late gaffe to claim the Bet At corbettsports.com Levy Board Handicap Chase.
Sue Smith's charge appeared in the clear two fences from home after game front-runner Bold Chief, the 4-1 favourite, wilted along the straight.
But Ryan Mania's partner met the penultimate obstacle all wrong, which gave runner-up Shotavodka a glimmer of a chance.
No Planning (16-1) swiftly regained momentum, though, and extended his advantage to three lengths at the line.
Smith said: "I've been struggling to find races for him and I think he goes to the field after this.
"Everything about this race suited him."
Witness In Court (7-2) had earlier looked sharp en route to a 13-length defeat of Kitegen in the Marie Turner Easter Bonnet Memorial Novices' Limited Handicap Chase.