I did a CPL over several years (well the TK, about 6 months for the flying) with no intention of being an airline pilot (and it worked!, I'm still not an airline pilot!).
I used CATS distance learning, studying a few evenings a week. It did require a lot of self discipline and by and large it was quite enjoyable - CATS material is comprehensive and readable, and I learned a lot which I've used in my day job as a boffin. As it happens however, CPL+CRI gets me 30ish hours of instructing a year, and some occasional ferry trips, so it was well worth the effort.
When it comes to the flying side, I regret spacing it out more than I do the TK - this ended up costing me a lot more money than it needed to, and if I did it again, I'd do the flying course intensively, to get it done in minimum (very expensive) hours.
It's not difficult any of it - but there is a hell of a lot of material, and it is a bit of a strain learning after years of PPLing to fly to CPL levels of accuracy.
G