What a strnage agenda. The idea of music being a chore is very alien to me !
A brief bit of research reveals playing an instrument reduces the change of dementia.
A word regarding your enjoyment of music:
This is akin to forcing yourself to play football when you'd rather just watch it, in order to get fit. It'll work but it's not a given that you'll enjoy it. But there's a difference: If you enjoy listening to music, try listening to yourself play. Make stuff up. Find where the bits are that you enjoy.
If you're able to do that, and find enjoyment in just playing, then you don't need to practice. Just play. play what you like, vary it, play that .. whatever. "play" is the keyword, not "work".
If you sit down and try to practice something regimentally, then yes that is boring. Are you deliberately choosing difficult exercises to test yourself? That might be your problem.
If you look for a favourite part of a song you're learning (eg for me it was the guitar solo at the end of Sultans of Swing, and Voodoo Chile by J. Hendrix), then you've got something to aim at which is more rewarding. Soon you'll realise that it's not just that part, but the context & how the music got there that's just as important. Next thing you've learnt the whole piece.
Or just noodling about and finding what the guitar has to offer you will work: you might end up writing a song.
All these will work the same on your brain, if indeed such cognitive improvements are generally the case, but going for something you want to achieve / hear is miles more fun and that means you're more likely to keep it up.