Having mountain biked the Speyside way, while there are some nice bits (Fochaber to Spey Bay in particular), overall I wasn't that keen, several bits where the council wasn't willing to fight with landowners for access, so well away from the river, or enclosed in old railway cuttings / commercial forest tracks i.e. the easiest line negotiate / promote/ use.
Dee, while the Deeside line mentioned above is away from the river, you can actually walk a lot of the riverbank on fishermans/local paths. Aberdeen to Peterculter is entirely on river bank (N, though there is one bit where a new barbed wire fence has just gone up - I'll be reporting to the local access officer), from there to Banchory would be on the Deeside line which is at times close to the river (though there are loops that can give good stretches of river walking where the line is away from the river (e.g. from Drum through Park, but they aren't efficient in terms of making progress west), Banchory to Portarch is partly bankside (S), then spruce forestry & minor roads, Aboyne to Ballater can predominantly done bankside (N) with the Deeside line for short bits (though its very nice that far west), You can also detour to do a circuit of Loch Kinnord and Burn o' Vat (from Dinnet), Intermittent river bank / road walking to Balmoral, where if the royals aren't in residence you can walk through to Ballochbuie (or do the Royal Cairns walk through the woods South of Balmoral if the big Hoose is occupied), cross to Invercauld and via estate tracks in Caledonian forest to the river and Marr Lodge, thence onwards to the source.
Its not perfect, a lot of the Dee Bankside paths above aren't sign-posted and are indistinct, or at times are field rigs, you just have to responsibly exercise your access rights, but in some ways is nicer for it. But if it doesn't work the Deeside line/minor roads are always relatively close.
If mad-focused and pedal-powered you could swim at Aberdeen beach, cycle, (deeside line and road) to Derry Lodge, followed by a run to make the top of Macdui within the day
Post edited at 14:25