I just went down to Preston. Got the 7 am train from Thurso, went via Edinburgh down to Preston. Got to Preston at 1715 then a quick Google tells me there are frequent trains from Preston to Blackpool, and the journey takes less than 30 minutes. Returning I get the 0941 train from Preston to Glasgow, megabus from Glasgow to Inverness and then the last train up. But you can also get trains to connect Glasgow to Inverness or come back via Edinburgh. I didn't check the trains from Blackpool to Preston but would assume they are also frequent earlier in the day. Hope this helps.
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