The 32-year-old carer finished seventh overall in the gruelling 38-miles ultra marathon.
Isobel, who is the reigning Scottish 100km champion, was quick to admit that the mile after mile of mud and hills had been a struggle. She said: "That's the hardest race I've ever run. Going over the Eildons with tired legs is hard and you've still got almost 18 miles to go."
Isobel crossed the finishing line in a time of 6.54.23.
Garscube's Sharon Law (7.04.12) was second lady home with Caroline McKay (7.16.19) from Edinburgh in third.
Over 1200 athletes from all over the country took part in Jedburgh Running Festival's three running races as well as the championship wheelchair and hand-cycle 10ks.
A record 689 names went down for the half marathon with a new high of 487 entries in for the 10k. But it was the gruelling 38-miles Three Peaks Ultra which grabbed most of the attention attracting 121 of Britain's top endurance runners.
Only six failed to finish the route which took in Maxton, St Boswells, the three Eildon peaks, Bowden and Newtown St Boswells. Craig Cunningham (5.34.23) from Portobello took the top honour with Richie Cunningham (5.39.08) from Carnegie in second, after taking a mile detour near the end, and Truro's Duncan Oakes (5.45.13) in third.
Craig said: "I was surprised when I was told I'd won as I knew Richie was in front of me. It's a shame he took the wrong turn but that's the way ultras are sometimes.
"It was really tough with the amount of mud but I still enjoyed the course."
Newly appointed Peeblesshire News Editor Ally McGilvray finished the ultra in 63rd position with a time of 9.01.06.
The tenth running of the half-marathon saw the four-year-old course record smashed by Ethiopian Yared Hagos. The former ice hockey star, who recently won the Invernesss half marathon, crossed the finishing line in 1.06.29.