an uphill struggle
A task that is very daunting from the outset and continues to be challenging. The incumbent is so popular that defeating her will be an uphill struggle. I'm terrible at math, so I have a real uphill struggle ahead of me if I want to improve my grade in Algebra.
an uphill ˈstruggle/ˈbattle/ˈtask
something that is difficult and takes a lot of effort over a long period of time: After the recent scandal, he faces an uphill struggle to win back public support before the next election.