I used to do this on my 60 mile commute, driving most of the way and cycling the final 12 miles or so.
I was lucky in that I was familiar with the village I parked in (Used to go out with a girl who lived there) and it just so happened to be an ideal commute distance with a suitable route.
My rationale was the same as yours; yes, it added time to the journey, but meant that I got some miles in during the week. It was easy enough on the way home to extend the ride too.
I can’t really help with parking location; perhaps look on Google Maps for a suitable industrial estate that will be empty when you get there, but probably fill with cars during the day. That, or a Park & Ride as suggested.
Preparation is the key to it working. I used to find that any excuse to get out of it and I would take it at 6 in the morning.
Making sure literally everything was ready to go, meant I was more likely to crawl out of bed.