Sounds a really nice idea. We've done both, but not on the same journey.
How will you carry your backpack when on the bike? Personally I wouldn't want to have it on my back while cycling although I know plenty do. You could use stretchy straps (bungees) to tie it to the back carrier - better if it was a nice compact shape rather than long and thin on a rigid frame, maybe. Something to think about, anyway.
You may be able to fix a bum bag (fannypack?) to the handlebars.
And your boots? I couldn't ride in my walking boots.
Make sure you have a bell and use it. So many cyclists come up behind walkers silently - perhaps they think the walker will hear their wheels or has eyes in the back of the head - and look offended if the walker takes a few moments to move aside. We have bells, and although it feels a bit rude and imperious to ring them as one approaches walkers, actually if you do it in time no-one needs check their pace - they glance over their shoulder (sometimes it takes more than one 'ting' for people to notice) and move to one side, and we the cyclists can slow down a bit and pass with a pleasant thank you or gracias.
And if walkers are coming towards a cyclist on a narrow path, there is no rule which says the walker has to be the one who stops on one side, possibly in the nettles or brambles. It's quite a cool game to play to see who will be the most polite and stop first, rather than who will be the most assertive and NOT stop!
Just a thought!