"...our garage gutters are leaking water all over the track and making it muddy."
How old is the garage? Has it been there for at least 20 years?
If it as least 20 years old then it is likely that you have gained the prescriptive right to have the gutters discharging water there regardless of who owns the land.
"But the garage is over 30m from the start of her driveway."
I'm a little confused by this (a diagram would help). I think that you own a 50m driveway over which your neighbour has a right of way to get to her own property. This is partly gravelled. The garage is near the part that is not gravelled.
There is then a 40m stretch beyond your driveway that is on the neighbours property?
She is now asking you to fill in a pothole on her land or yours?
If it's on her land then it's down to her to sort it out.
If it's on the part of the driveway that is on your land but she has a right of way over then it's still down to her to maintain it. You don't have to maintain it as the landowner.
However, if the water is discharging onto her property and the garage has not been there for 20 years then she may have a claim to get you to stop discharging water onto her property.