Yes, sadly Scotland's forests have been decimated over hundreds of years, and especially around the time of the first world war. After that, fast-growing Sitka spruces were planted, which were no use for the native wildlife. Native forests are being re-planted now, but it will take hundreds of years for them to mature.
And yes, you have to build up your leg muscles quite a lot to climb the hills. Or just go up them really slowly!